test_util: Implement test for is_gitlab_instance

[sonic2kk]

This PR implements a test for the is_gitlab_instance utility function.

We test against each element in our GITLAB_API list constant using PyTest's Parametrize feature. Currently we only have one entry in this list, but if we had more in future, this would allow us to check to make sure if we add one that it this function should always work with all items in that list. We also then check a failing test case with the GitHub API to ensure that the function returns False when we expect it to.

Thanks!

[DavidoTek]

Awesome to see that the number of tests is growing, thanks!

PS: I may be a bit slow at reviewing currently due to some other projects...

[sonic2kk]

No problem! I spent a lot of time trying to figure out PyTest and good testing standards / practices, and when I finally felt like I understood, I decide to try and make good on my promise from a while back to actually write some tests.

The PRs for the tests are not testing functionality that I foresee changing, so there's no need to hurry on review :-) I did test merging them into main locally and it seems there'll be conflicts at some point so I'll resolve those once we get to them!