tests: Add a manual integration test for OstreeRepoFinderMount

tests: Add a manual integration test for OstreeRepoFinderMount

Test an end-to-end flow of pulling refs from an online
repository → local OS repository → create a USB stick of
them → pull to a local OS repository on another machine.

This is a manual test, as it requires a throwaway USB stick which the
test can format as ext4 or vfat to test the flow works with both file
systems.

Run it as:

MOUNT_INTEGRATION_DEV=/dev/sdb1 make check \
  TESTS=tests/test-repo-finder-mount-integration.sh

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1203
Approved by: cgwalters