Add unit test for varargs icw @TargetType

* `@TargetType` should always be a class. Perhaps this works out of the box. However a test case would be a good idea.
* `@MappingTarget` should work on handwritten method. Add test case to verify this.

see also #1550

Start by reading issue #1550 and the repository's existing tests for @TargetType and @MappingTarget. Add unit coverage for varargs methods, confirming that @TargetType accepts a class and that @MappingTarget works on handwritten methods; the tests should pass and document both behaviors.

Written by the indexing model from the issue text.