copies-rust: add a macro-based unit-testing framework `compare_values`, `merge_copies_dict`, and `CombineChangesetCopies` are APIs whose signatures involve non-trivial types. Calling them directly in unit tests would involve a lot of verbose setup code that obscures the meaningful parts of a given test case. This adds a macro-based test-harness with pseudo-syntax to tersely create arguments and expected return values in the correct types. For now there is only one (not particularly meaningful) test case per tested function, just to exercize the macros. Differential Revision: https://phab.mercurial-scm.org/D10071