Make tests more robust

[yuvipanda]

• Don't require connection to the internet for api tests to run (I was just on a plane!). Instead, we use the helper classes used in test_gitpuller for making arbitrary temporary git repos in test_api as well
• Don't specify hardcoded names when creating our test git repos - instead, create truly temporary directories.
• Stop swallowing exceptions that might occur when trying to fetch list of tags / branches in a remote. Without this commit, we were swallowing the output of the git command itself

[consideRatio]

This makes sense! It's great that you consider the helpfulness of error messages, it sure matters when it matters :)

Linkback to other issues regarding error messages:

• https://github.com/jupyterhub/nbgitpuller/issues/267
• https://github.com/jupyterhub/nbgitpuller/issues/198