Limit all test suite selection operations to #qunit-fixture

[gibson042]

Cf. jquery/jquery/issues/2880

This issue should close when one of the following two conditions are satisfied:

• Someone adds a comment certifying that they have examined all unit tests and found no selections scoped so broadly that they could match QUnit output.
• A commit lands on the master branch claiming to correct all such selections (e.g., by either scoping more tightly or—for those that are explicitly document-scope—reducing with jQuery#filter( "#qunit-fixture *" ) before comparing to expected results).

[mgol]

Not going to happen now that we plan to archive Sizzle soon. For jQuery, we have a roadmap item at https://github.com/jquery/jquery/wiki/Roadmap#ongoing-work-and-ideas