tests: Support TEST_SKIP_CLEANUP=err
tests: Support TEST_SKIP_CLEANUP=err

I find myself often wanting to debug interactively failing tests.
This makes it more convenient to keep around the temporary directories
just for those tests, rather than accumulating tons of tempdirs from
the successful tests as well.

Closes: #588
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree