Note that whatwg-url tests should not be modified

[gibfahn]

• Version: master
• Subsystem: test, url

Add a comment to the whatwg-url tests to document that we should not modify them since they are imported tests.

Refs: #12669 (comment) and #12743 (comment)

cc/ @Trott @joyeecheung @cjihrig

[joyeecheung]

Tips: the whatwg-url tests are test/parallel/test-whatwg-*.js, the meaning of the wrapper can be found in https://github.com/nodejs/node/blob/master/doc/guides/writing-tests.md#imported-tests

[refack]

@gibfahn added "Since they are Imported Tests" to first comment if that's Ok.

[Trott]

Additional information:

• It's not the entire test file that shouldn't be modified, but most of the test file. Most of each test is copied from the canonical WPT tests and that part should not be modified. While we might accept a blanket "do not modify this test" comment, it would be preferable if the comment were clear about what part of the test should not be modified.