Make CascadingStylesheetsTest 3500% faster

While working on #352951: Make JS & CSS Preprocessing Pluggable, I got very much fed up with the extreme slowness of CascadingStylesheetsTest.php.

Difference

• Before: 281 seconds
• After: 8 seconds
• (That's 35.125 times faster, or only 2.85% of the original test running time.)

It used WebTestBase for historical reasons, while it could easily use DrupalUnitTestBase now.

Ideally, this would be a PHPUnit test, but that's not technically possible right now. This will at least make it a whole lot faster, while retaining the same test coverage.

Changes

I changed as little as possible:

• DrupalWebTestBase -> DrupalUnitTestBase
• Removed testRenderInlineFullPage() since it was rather pointless: it tests adding inline CSS, but that's already tested in two other tests, and in fact to do so it used the PHP filter of all things…
• Instead of relying on a certain page callback in common_test module, which would indeed need WebTestBase, I just moved the relevant parts of common_test.module into the test itself (2 lines of code).