Enable phpcs for the tests folder

[kevinfodness]

Enables phpcs checks on files in the tests directory, because we should be enforcing coding standards everywhere, including when writing tests. Some notes on the change:

• This is more for code formatting than it is for security/performance warnings, which will need to be selectively ignored in tests (e.g., when running a core function as part of a test fixture that phpcs will complain about, like flushing permalinks). However, this is easily done as an inline // phpcs:ignore within the test file.
• This PR changes the naming convention of test classes back to class-* to make the phpcs tests pass, and updates the phpunit config to look for tests in all PHP files in the tests directory. PHPUnit is pretty smart about how it does this, and this change did not result in any difference in execution time for sample tests in this repo, even by looking at a more permissive set of files. See relevant discussion here on where the test- prefix came from and how it isn't an official PHPUnit standard, despite being in wide use in WP projects scaffolded using WP-CLI: WordPress.Files.FileName.InvalidClassFileName for tests/test-sample.php WordPress/WordPress-Coding-Standards#882

[anubisthejackle]

Looks good to me.

One thing we could do to help alleviate some of the pain of ignoring PHPCS issues in tests is put together a list of common safe exclusions for tests and add those to the .phpcs.xml file. I don't think that needs to happen today, but it might be something to consider as we find functions we're repeatedly ignoring.