Move and rename the existing git-interface tests such that tests for
other scripts can be added easily.

In particular, the following changes are made:

* Move the existing tests from git-interface/test/ to test/.
* Rename t0001-auth.sh to t1100-git-auth.sh.
* Rename t0002-serve.sh to t1200-git-serve.sh.
* Rename t0003-update.sh to t1300-git-update.sh.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>