tests/compose: Add RPMOSTREE_TMPDIR_LOCATION

This allows one to run the tests from a container using overlay +
SELinux protection by running the actual compose into a non-overlay
bind-mount. Otherwise, we'll hit `ENOTSUP` when trying to set labels on
various checkouts.

Closes: #1829
Approved by: cgwalters

tests/compose-tests/libcomposetest.sh

@@ -1,5 +1,5 @@
 dn=$(cd $(dirname $0) && pwd)
-test_tmpdir=$(mktemp -d /var/tmp/rpm-ostree-compose-test.XXXXXX)
+test_tmpdir=$(mktemp -d ${RPMOSTREE_TMPDIR_LOCATION:-/var/tmp}/rpm-ostree-compose-test.XXXXXX)
 touch ${test_tmpdir}/.test
 trap _cleanup_tmpdir EXIT
 cd ${test_tmpdir}