xfs: document the testing plan for online fsck

Start the third chapter of the online fsck design documentation.  This
covers the testing plan to make sure that both online and offline fsck
can detect arbitrary problems and correct them without making things
worse.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>