bpo-38270: Fix indentation of test_hmac assertions (GH-17446)

Since https://github.com/python/cpython/commit/c64a1a61e6fc542cada40eb069a239317e1af36e two assertions were indented and thus ignored when running test_hmac.

This PR fixes it. As the change is quite trivial I didn't add a NEWS entry.


https://bugs.python.org/issue38270