Test task is skipped when there are no tests. Should resolve recent CI failures.

1. Reworked my recent changes related to testCompile and testRuntime resolution for 'no tests' scenario. Using @SkipWhenEmpty feature breaks certain existing behavior (the validation of the test.single matching result). Now at least when test.single is not used, we have the desired behavior and the test task is skipped (and testCompile/testRuntime configurations not resolved). Reverted the annotation change on the getCandidateClassFiles() to what it was before my changes.
2. In some integration tests I needed to add a dummy test class file so that the test task is not skipped.