Unify integration/acceptance test formats across (Scala compiler) builtin cases & plugin cases

[tribbloid]

As we gradually push stable features from this plugin into Scala compiler, we expect test cases to be also migrated smoothly.

Unfortunately the newer builtin test cases were written to resemble partest in Scala project, which is quite different from the older plugin test cases. The next version should unify them by upgrading both to adhere to partest convention

@tek this should be a maintenance patch that has no impact on all test outputs