tests: (lsfd) refine the pattern for comparing the output of the commands

tests: (lsfd) refine the pattern for comparing the output of the commands

When comparing the output of lsfd and findmnt commands, I passed just
"$MNTID" to grep for filtering the output of findmnt. It was too relaxed.

It should be "$MNTID\b"; a space after $MNTID was needed.

https://github.com/util-linux/util-linux/issues/1511#issuecomment-1029968595
shows the output triggering the bug of the test case.

Reported-by: Chris Hofstaedtler <zeha@debian.org>
Reported-by: Anatoly Pugachev <matorola@gmail.com>
Signed-off-by: Masatake YAMATO <yamato@redhat.com>