With out this, empty directories in the refs/heads directory just
grow unconditionally.

Enhance the test which checks removal of the extract directories
with an additional check that the ref directories are cleaned up
when removing artifacts.

When used in a child process in BuildStream, this should be
used instead of tempfile.NamedTemporaryFile() directly, otherwise
we fail to cleanup the file on SIGTERM.

The direct usage of tempfile.TemporaryDirectory() and
tempfile.NamedTemporaryFile() here causes leakage of any temporary
data when the process operating on temporary data is terminated
with SIGTERM.

Using the utilities ensures that trash is not left behind in
~/.cache/buildstream/artifacts/tmp when the user terminates
BuildStream with ^C.