[core] Fix coverage tests

The original coverage tests used yui-core-coverage and loaded the rest of yui-base by inserting script tags. This seems to have been set up that way to get more granular coverage information than just one entry for yui-base, but it is causing YUI.add to fail in certain cases because Loader is not yet available. This changes the coverage test to do the same as the regular unit tests.

@reid @caridy does this make sense to you? Is there anything you can think of that we wouldn't be testing by making this change?