bpo-44048: Fix two hashlib test cases under FIPS mode (GH-26470) (GH-26531)

test_disallow_instantiation and test_readonly_types try to test all the available
digests, however under FIPS mode, while the algorithms are available, trying to use
them will fail with a ValueError.
(cherry picked from commit a46c220edc5cf716d0b71eb80ac29ecdb4ebb430)

Co-authored-by: stratakis <cstratak@redhat.com>

Co-authored-by: stratakis <cstratak@redhat.com>