Invert unless..else conditions on JRuby checks
Simplify plugin tests a bit, leave the regexp work for minitest
Simplify path setup
Use default argument when testing generators without the need for extra args
Avoid defining the test if it does not need to when not on JRuby
Properly assert for the expected messages

The message passed to Minitest's assert_raise is used as output in case
the assertion fails, but we can test against the exact message by using
the actual exception object that is returned from the assert_raise call.