populate: cache scratch metadata images

Create a helper function to create a populated FS image and dump the
metadata into a file on the test device, with the purpose of allowing
future (fuzzer) invocations of _populate_fs use the cached metadata to
save time.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>