btrfs-progs: Add corrupted leaf repair testcase

Add corrupted leaf repair testcase with small test framework change.

Small framework modification includes the following:
1. Add support for custom script to create test image
This can great reduce image size since btrfs-image dump with -c9
option can produce much smaller image than binary dump.

2. Don't check btrfsck --repair return value
Since corrupted-leaf case involves a lot of different repairing parts,
some of them are even called before corrupted-leaf repair, so they
may report error when doing repair.
But the repair result is a full btrfsck OK image.
So the return value of btrfsck --repair can be ignored.