tests: check for relabeling rather than overlay

tests: check for relabeling rather than overlay

Instead of checking for overlayfs, let's explicitly check for our
ability to relabel files since we now have a `libtest` function to do
this. Also port that logic to `libostreetest`.

Note that overlayfs *does* allow manipulating user xattrs. So ideally,
we should break down `OSTREE_NO_XATTRS` further to distinguish between
tests that use bare repos from other modes.

We check the current directory instead of `/` so that developers can
just point `TEST_TMPDIR` to a non-overlayfs mount point when hacking
from a container.

Closes: #1170
Approved by: cgwalters