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.
Why not pass `cordova.require('org.apache.cordova.test-framework.cdvtests')` here, and change the definition of `toggleTestEnabled` to accept "tests" always as second parameter, instead of the awkward optional second getter function?