HMAC's bigmem tests for `hmac.digest` are incorrect

[picnixz]

Bug report

Bug description:

The bigmem test for hmac.digest actually do NOT exercise the actual hashing as the function is mocked.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-155430: fix hmac.digest bigmem test #155431
• [3.15] gh-155430: fix hmac.digest bigmem test (GH-155431) #155846
• [3.14] gh-155430: fix hmac.digest bigmem test (GH-155431) #155847