tests: add return type hints to tests

[mattcristal]

Hey, I just made a Pull Request!

Description

I have added the return type hints to some test functions.

Motivation and Context

Taking advantage of type hints, this contributes to make the test functions more clear and readable.

Have you tested this? If so, how?

I have run, locally, on macOS:

python -m pytest

All tests are passing. Overall, there is no change in behavior.

Checklist for PR author(s)

• Changes are covered by unit tests (no major decrease in code coverage %).
• All tests pass.
• Docstring coverage is 100% via tox -e docs or interrogate -c pyproject.toml (I mean, we should set a good example 😄).
• Updates to documentation:
  • Document any relevant additions/changes in README.rst.
  • Manually update both the README.rst and docs/index.rst for any new/changed CLI flags.
  • Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note