fstests: add helper _scratch_shutdown_and_syncfs

fstests: add helper _scratch_shutdown_and_syncfs

Test xfs/546 has to chain syncfs after shutdown and cannot
use the _scratch_shutdown helper, because after shutdown a fd
cannot be opened to execute syncfs on.

The xfs_io command of chaining syncfs after shutdown is rather
more complex to execute in the derived overlayfs test overlay/087.

Add a helper to abstract this complexity from test writers.
Add a _require statement to match.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>