tests: Convert remaining tests which write data into the repo

This MR is the final part in my quest to improve our testing strategy to not write any data into the repo itself anymore. It converts all such uses I found and adds a CI hack which will make most of our directories read-only before executing our tests. As a result, any test which would write data into the repository itself will now fail with a permission denied error. This should help avoiding this antipattern.

Please note that there is a change in behaviour in the first commit 95be844f. Previously, we would've written hook logs into the current working directory in case GITALY_LOG_DIR was unset. This shouldn't happen in any real setups, but even if it did it's probably not the intention to write logfiles wherever we stand.