gh-130149: refactor tests for HMAC (#130150)

Since we plan to introduce a built-in implementation for HMAC based on HACL*,
it becomes important for the HMAC tests to be flexible enough to avoid code
duplication.

In addition to the new layout based on mixin classes, we extend test coverage by
also testing the `__repr__` of HMAC objects and the HMAC one-shot functions.

We also fix the import to `_sha256` which, since gh-101924, resulted in some tests being
skipped as the module is no more available (its content was moved to the `_sha2` module).