gh-130149: cleanup refactorization of `test_hmac.py` (#131318)

New features:

* refactor `hashlib_helper.requires_hashdigest` in prevision of a future
  `hashlib_helper.requires_builtin_hashdigest` for built-in hashes only
* add `hashlib_helper.requires_openssl_hashdigest` to request OpenSSL
   hashes, assuming that `_hashlib` exists.

Refactoring:

* split hmac.copy() test by implementation
* update how algorithms are discovered for RFC test cases
* simplify how OpenSSL hash digests are requested
* refactor hexdigest tests for RFC test vectors
* typo fix: `assert_hmac_hexdigest_by_new` -> `assert_hmac_hexdigest_by_name`

Improvements:

* strengthen contract on `hmac_new_by_name` and `hmac_digest_by_name`
* rename mixin classes to better match their responsibility