Add jQuery UI Core as a dependency for qUnit tests

[30919] added more qUnit tests for the Customizer. One of the dependencies for these tests is qUnit. Otherwise we end up with an error due to an unsupported pseudo.

https://travis-ci.org/aaronjorbin/develop.wordpress/jobs/44873635#L425 shows an example of the failing test.

Props ocean90.
see #30701.
Fixes #30754.



git-svn-id: http://develop.svn.wordpress.org/trunk@30989 602fd350-edb4-49c9-b593-d223f7449a82