Make JavaScriptTest 3200% faster

(This is the sister issue of #2026255: Make CascadingStylesheetsTest 3500% faster.)

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

Difference

• Before: 610 seconds (10 min 10 sec)
• After: 19 seconds
• (That's 32.105 times faster, or only 3.11% 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 the testing of the presence of drupalSettings.currentPath in the actual rendered HTML; it's already being tested higher up in testHeaderSetting() anyway.
• 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).