Silently create test_cache if it does not exist

[Casper-Guo]

If you are running tests locally and the test_cache directory hasn't been created before, enable_cache throws the following error:

NotADirectoryError: Cache directory does not exist! Please check for typos or create it first.

This is counter-intuitive as most contributors would have a cache enabled somewhere already.

Opening the PR to propose that this be handled silently (or with log output). The specific cache location might need to be changed to be consistent with the general fastf1 cache

[theOehrly]

I think it is fine to just create this local directory. So no need to use a different location for it.

But should we also add the "test_cache" folder to the .gitignore then? Else people might accidentally commit data from it.

[Casper-Guo]

.gitignore ignores all directories ending in "cache" so this is already covered

[theOehrly]

Oh right, then this looks good.