CB-7459 Allow automatic tests to be run for specific plugin(s)

Implementation for https://issues.apache.org/jira/browse/CB-7459

See comments in commits for more details.
Perhaps we don't need these two methods now.

Can just use:
`cdvtests.tests[api].getEnabled()` directly.  That also has the advantage of not creating a default TestModule if you pass an invalid api.  This way, only the cdvtest `init()` function can create test modules.