[TST] Classy test data fixed to classy v2.9.4

[itrharrison]

Description

This PR updates the classy test data to be from a specific documented version, rather than the previous test data where it was unclear. I have also checked that this test data is O(10-3) compatible with camb.

I have also added a skipif to the test based on the classy version. We could think harder about the expected stability of the result between classy versions.

Checklist

• [X] Follow the Contributor Guidelines
• [X] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [X] Assign someone from the infrastructure team to review this pull request