enhancement: ability to exclude gRPC impls in a test case pattern

[jhump]

When tests are run against the gRPC client or server implementation (vs. against the standard reference client or server), the test case permutation name gets an extra (grpc client impl) or (grpc server impl) element in the name. For test case permutations run against the standard reference implementation, there is no extra element. This makes it a challenge to create a wildcard pattern that includes only the reference implementation permutations (i.e. all names that do not contain (grpc client impl) or (grpc server impl), which can't currently be expressed with wildcard syntax).

One way to address this would be to always include an element in the permutation name and have it vary between impl:grpc and impl:reference. That way **/impl:reference/** is an easy way to include only the reference implementation permutations.