Fix failure in test_checkpoint_activations_norm.py

[maksverver]

This test expected to fail with mixed_precision == 'fp16' but it works just fine on my system (and then the test fails because the expected exception is not thrown). I suspect the test is out of date.

What does this PR do?

Fixes a test failure.

Before submitting

• Did you have fun?
  • Not really
• Did you read the contributor guideline?
  • Yes
• Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
  • No
• Did you make sure to update the docs?
  • N/A
• Did you write any new necessary tests?
  • N/A
• Did you update the changelog? (if needed)
  • No, because I didn't change any production code, only tests.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.