### Motivation and Context
This PR slims down the number of integration tests running against AOAI
and OAI models - reducing time and resources spent running them.
Additional justification for slimming down these tests is to reduce the
chance of throttling from these endpoints (leads to unstable integration
tests).

The scenarios previously being covered are already handled using HF
models which are much less expensive to test against and do not run the
risk of throttling.


### Description
- added a top-level test `conftest.py` to handle kernel creation and
OpenAI model secret handling. I had originally intended `create_kernel`
to do more than just create a kernel, but additional setup wasn't
needed. I've left the pytest fixture though since other fixtures can
call it, and using it in tests can making importing Kernel from
semantic_kernel unncessary.
- added a completions `conftest.py` to set up completions tests. For
example, setup_hf_text_completion_function allows for testing
text2text_generation and text_generation models using the same test
file.
- Common test code is now in pytest fixtures instead of common methods.
- For a number of the completion tests, I have broken out the asserts to
individual tests instead of running one giant test. This makes it easier
to identify regressions in specific patterns around invoking skills.
- Added retry logic to conversationSummarySkill
- renamed tests to be more descriptive.