btrfs-progs: tests: sort image files

If a test has several images let filenames enforce a particular order of
checks if desired.

Signed-off-by: David Sterba <dsterba@suse.cz>

tests/common

@@ -52,7 +52,7 @@ check_all_images()
 	for image in $(find $dir \( -iname '*.img' -o	\
 				-iname '*.img.xz' -o 	\
 				-iname '*.raw' -o 	\
-				-iname '*.raw.xz' \) )
+				-iname '*.raw.xz' \) | sort)
 	do
 		cleanme=
 		case "$image" in