[tests] use core qunit-fixture properly

[tests] use core qunit-fixture properly
QUnit provides a <div id="qunit-fixture"> by default that is cleared out after each test.

* Switch tests to append to the provided #qunit-fixture instead of appending to the body.
* Remove redundant .remove() calls, which now happens automatically
* Fix jquery.textSelection.test to not cause dozens of elements to be appended to the body with duplicate IDs, which was causing WebFonts test suite to fail (since the DOM can't select multiple elements with the same DOM). (Fixes r92923, r100391)