tests: skip cc tests if plugin is disabled
In our internal CI, the line21 test case often fails intermittently (and not sure why). There does not appear to be any way to selectively disable test cases at runtime. Since we don't personally use the closedcaption elements, this will allow us to disable it at compile time.
Either way, our issue and motivation is mostly irrelevant since disabling the closedcaption plugin elements should also disable their tests.
Thus, skip the closedcaption plugin element tests if the closedcaption plugin option is disabled at compile time (i.e. -Dclosedcaption=disabled).