btrfs-progs: convert-tests: Add test case for backup superblock migration

btrfs-progs: convert-tests: Add test case for backup superblock migration

New convert introduced simpler chunk/extent allocation algorithm, at the
cost of complex backup superblock migration codes.

Use specially built ext2 images to test if btrfs-convert can convert and
rollback images without problem.

All these special ext2 image have blocks/holes across 2nd btrfs backup
superblock.

The naming of test image is like the following:
	|<------superblock migration range----->|
	64M					64M + 64K
|-Data--|-Data--|/Hole//|-Data--|/Hole//|-Data--|--Data--| = drdhdhdrd

These test cases should check all typical layouts and make sure new
convert works.

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