add test for completion logprobs

[mattf]

What does this PR do?

adds a test for the completion api's logprobs parameter

tbd which providers pass this test

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.