This is a prerequisite of implementing GMAC in RustCrypto/MACs#1

As GMAC is nonce-based it cannot implement KeyInit and thus the current mac_test function cannot work for it. However, it doesn't make sense to build significant logic for a single special MAC. So, this breaks mac_test into two parts:

1. A compatibility part (same name of mac_test) that constructs a Mac that implements KeyInit and then passes it to raw_mac_test.
2. raw_mac_test which is what actually does the testing but simply clones the supplied MAC.

This way all "normal" MACs can keep being tested the same way they always have but "special" MACs can take advantage of the shared test logic.

I did not modify reset_mac_test because nonce-based MACs like GMAC cannot be safely reset, so the refactoring would not have been worth it.

I am not passing in the existing MacTestVector structure, because it isn't applicable to nonce-based MACs and we only need input and tag so they are easy enough to pass independently.

I have manually tested this by redirecting the cmac tests to use my local changes. I also had my new gmac tests use the new logic successfully.