gitignore: Ignore gitlab-shell data created by tests

We've got some tests which set up gitlab-shell, which includes both writing a config file as well as writing the secret. We typically do so in the respective test's testdata directory, but that directory isn't generally ignored by our gitignore. Naturally enough, there's been a file which has been added by accident. Furthermore, we still have two gitkeep files around for the hooks directy, which isn't used anymore today.

This commit thus removes all testdata/gitlab-shell files and adds it to our gitignore.