docs: add usage examples for unittest.TestCase

[serl]

Description

Adding documentation for unittest.TestCase. Everything works already and pytest experts know already what to do, but for casual users migrating from snapshottest this may be very useful.

Related Issues

• Closes Unable to pass snapshot argument to custom test class methods #682, which asked for native unittest support. It is already closed but let's say that's very closed after this.

Checklist

• This PR has sufficient documentation.
• This PR has sufficient test coverage.
• This PR title satisfies semantic convention.

Additional Comments

No additional comments.

[noahnu]

@all-contributors add @serl for docs

[allcontributors]

@noahnu

I've put up a pull request to add @serl! 🎉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main    #1038   +/-   ##
=======================================
  Coverage   97.29%   97.29%           
=======================================
  Files          21       21           
  Lines        1666     1666           
=======================================
  Hits         1621     1621           
  Misses         45       45           

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