[FEA] Test against stable tags for narwhals

[vyasr]

Is your feature request related to a problem? Please describe.
We now have CI jobs that clone the test suites from narwhals. This is very helpful to use for maintaining coverage of the relevant APIs. We would like to eventually move to a model where we can require that these tests pass consistently, but at the moment these are not required CI jobs due to potential instability from upstream changes.

Describe the solution you'd like
For pandas tests, we check out a stable branch of the code prior to running tests. That branch is controlled by the pandas version that we install into the testing environment, which in turn is managed by dependencies.yaml. To improve the stability of our narwhals tests, instead of cloning the latest of any branch (even their stable branch) we should follow the same pattern.