selftests: always check mask returned by statmount(2)

selftests: always check mask returned by statmount(2)

STATMOUNT_MNT_OPTS can actually be missing if there are no options.  This
is a change of behavior since 75ead69a7173 ("fs: don't let statmount return
empty strings").

The other checks shouldn't actually trigger, but add them for correctness
and for easier debugging if the test fails.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250129160641.35485-1-mszeredi@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>