Update docs to explain how WP_Mock TestCase is preferable to extend than PhpUnit's

[unfulvio-godaddy]

Summary

Updates docs to make it clear that implementations should use WP_Mock own TestCase, ideally, or at least provide teardown overrides.

Closes: #238

Contributor checklist

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly
• I have added tests to cover changes introduced by this pull request
• All new and existing tests pass

Reviewer checklist

• Code changes review
• Documentation changes review
• Doc examples should always reference WP_Mock own TestCase
• Unit tests pass
• Static analysis passes

[coveralls]

coverage: 66.445% (+0.1%) from 66.312%
when pulling 1517e83 on issue/238-update-docs-when-test-case-not-extended
into 4249c87 on trunk.

[BrianHenryIE]

FTR: I don't use it by extending TestCase, I just run the global WP_Mock::setUp(); and WP_Mock::tearDown(); and it works great for me!

https://github.com/BrianHenryIE/bh-wp-autologin-urls/blob/ace7784c2c8b1427f3a96db5a1e9daa29dbb852e/tests/unit/api/class-api-unit-Test.php#L24-L34

[unfulvio-godaddy]

@BrianHenryIE that is correct - as mentioned in the docs we are updating in this PR, you can override precisely those two methods to achieve the same thing. However, it's simpler to tell people to extend WP_Mock own test case (which in turn extends PHPUnit's own test case anyway). We mention the alternative you are using in the docs as per this PR. Feel free to comment further if you think we should add anything else, thank you!

[ajaynes-godaddy]

LGTM! Big improvement.