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 do you pass getTests() when `toggleTestEnabled` will just do a cordova.require('org.apache.cordova.test-framework.cdvtests'); internally?  cordova.require does internal caching so its not an expensive operation.