…t#676)

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### Description of PR
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Summary:
Fixed nightly test failures in `test_ecn_config_update.py` on some
platforms due to invalid new values for `green_min_threshold` or
`green_max_threshold` fields.

### Type of change

<!--
- Fill x for your type of change.
- e.g.
- [x] Bug fix
-->

- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [ ] New Test case
    - [ ] Skipped for non-supported platforms
- [x] Test case improvement

### Approach
#### What is the motivation for this PR?
Nightly test failures in `test_ecn_config_update.py` on some platforms.

#### How did you do it?
When changing the `green_min_threshold` or `green_max_threshold` field
of a WRED profile, we must ensure that the new values are valid. In
particular, these conditions should always be true:
1. `green_min_threshold <= green_max_threshold`.
2. `green_max_threshold <= buffer pool size`
On some platforms (e.g., Mellanox), `green_max_threshold` must be a
multiple of a certain number (unit size). If it is not provided as a
multiple of unit size in the config, then SAI rounds-up
`green_max_threshold` to the first multiple of unit size. The rounded-up
value must still be less than buffer pool size. Otherwise, the ASIC will
reject the new value.
To ensure these constraints are not violated, we never increase
`green_max_threshold`. A new functions named `determine_delta_values`
will decide how much each field should change based on fields that are
changed in the test and current field values in the WRED profile.

#### How did you verify/test it?
Ran the tests on a Mellanox device on which the tests previously failed.
Here are the test results:
```
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_min_threshold] PASSED                                         [ 25%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_max_threshold] PASSED                                         [ 50%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_drop_probability] PASSED                                      [ 75%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_min_threshold,green_max_threshold,green_drop_probability] PASSED [100%]
```

#### Any platform specific information?
No. The test improvement is generic and applies to all platforms.

#### Supported testbed topology if it's a new test case?
N/A

### Documentation
<!--
(If it's a new feature, new test case)
Did you update documentation/Wiki relevant to your implementation?
Link to the wiki page?
-->
N/A

---------

Signed-off-by: Mahdi Ramezani <mramezani@microsoft.com>

…t#676)

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Please make sure you've read and understood our contributing guidelines;
https://github.com/sonic-net/SONiC/blob/gh-pages/CONTRIBUTING.md

Please provide following information to help code review process a bit
easier:
-->
### Description of PR
<!--
- Please include a summary of the change and which issue is fixed.
- Please also include relevant motivation and context. Where should
reviewer start? background context?
- List any dependencies that are required for this change.
-->

Summary:
Fixed nightly test failures in `test_ecn_config_update.py` on some
platforms due to invalid new values for `green_min_threshold` or
`green_max_threshold` fields.

### Type of change

<!--
- Fill x for your type of change.
- e.g.
- [x] Bug fix
-->

- [ ] Bug fix
- [ ] Testbed and Framework(new/improvement)
- [ ] New Test case
    - [ ] Skipped for non-supported platforms
- [x] Test case improvement

### Approach
#### What is the motivation for this PR?
Nightly test failures in `test_ecn_config_update.py` on some platforms.

#### How did you do it?
When changing the `green_min_threshold` or `green_max_threshold` field
of a WRED profile, we must ensure that the new values are valid. In
particular, these conditions should always be true:
1. `green_min_threshold <= green_max_threshold`.
2. `green_max_threshold <= buffer pool size`
On some platforms (e.g., Mellanox), `green_max_threshold` must be a
multiple of a certain number (unit size). If it is not provided as a
multiple of unit size in the config, then SAI rounds-up
`green_max_threshold` to the first multiple of unit size. The rounded-up
value must still be less than buffer pool size. Otherwise, the ASIC will
reject the new value.
To ensure these constraints are not violated, we never increase
`green_max_threshold`. A new functions named `determine_delta_values`
will decide how much each field should change based on fields that are
changed in the test and current field values in the WRED profile.

#### How did you verify/test it?
Ran the tests on a Mellanox device on which the tests previously failed.
Here are the test results:
```
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_min_threshold] PASSED                                         [ 25%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_max_threshold] PASSED                                         [ 50%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_drop_probability] PASSED                                      [ 75%]
generic_config_updater/test_ecn_config_update.py::test_ecn_config_updates[replace-green_min_threshold,green_max_threshold,green_drop_probability] PASSED [100%]
```

#### Any platform specific information?
No. The test improvement is generic and applies to all platforms.

#### Supported testbed topology if it's a new test case?
N/A

### Documentation
<!--
(If it's a new feature, new test case)
Did you update documentation/Wiki relevant to your implementation?
Link to the wiki page?
-->
N/A

---------

Signed-off-by: Mahdi Ramezani <mramezani@microsoft.com>