Fixes #7483 (comment)

Changes

• Avoid double serialization in test code that exercises RPC endpoints.

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

Updated tests accordingly without changing existing behavior.

Documentation

Requires documentation update

• Yes
• No

Requires explanation in Release Notes

• Yes
• No

Remarks

During #7483 we found out that our testing code could potentially result in double serialization of objects. For example, instead of

{ "foo": 42 }

we would get

"{\"foo\": 42 }"

This PR addresses this issue avoiding double serialization. This also reduces the code in several test cases where we no longer need to instantiate a JSON serializer.

Due to the goal of this PR it should be reviewed independently of other refactoring PRs.