btrfs-progs: tests: Allow check test to repair in lowmem mode for certain errors

btrfs-progs: tests: Allow check test to repair in lowmem mode for certain errors

Since lowmem mode can repair certain corruptions (mostly in fs tree),
insert a beacon into each fsck test cases to allow some of them be
tested in lowmem mode.

With this patch, fsck option override will check the beacon file
".lowmem_repairable" in the same directory of the test image, and if the
beacon exists, then it will also run lowmem mode repair to repair the
image.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>