Move testdata into unified global and temporary directory

Our tests are quite happy to generate loads and loads of data all over the place. While the cruft isn't as visible in Gitaly's repo itself due to our ignore rules, it's certainly visible inside of /tmp. After a few testruns, it'll contain hundreds if not even thousands of files. This MR ends that by instead creating a single unified global temporary directory which is used by most of our testing infrastructure.

This MR doesn't yet go all the way, and there's still tests and infrastructure which sets up its own things or uses the Gitaly repo. Most importantly, this also includes the test storage we created. But these changes would be a little less mechanical, so I wanted to instead tackle them in a follow up MR at a later point. The MR is already large enough, even though most of it is indeed mechanical.