repository: Fix test writing into the source directory

I've recently noticed that tests sometimes leave behind an empty full directory in the repository service package. This is caused by a bug in one of the tests for cloneFromURLCommand(), which clones a repository into a relative path. As the command's context is immediately getting cancelled, whether or not the directory exists is subject to a race depending on we kill git-clone(1) before it's created the target dir.

Fix this bug by instead cloning to a temporary directory. Parallelize the test while at it.