btrfs-progs: tests: introduce test dev mount helpers

btrfs-progs: tests: introduce test dev mount helpers

mount command in old system can not add "-o loop" option automatically
for a loop device, and make following test 013-extent-tree-rebuild fail.

Considering that $TEST_DEV can be block or loop device, we need to determine
our mount option in a condition for both case.

Introduce a wrapper that will add the loop options if needed.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>