btrfs-progs: tests: Add '-o loop' to mount command line in convert-tests.sh

btrfs-progs: tests: Add '-o loop' to mount command line in convert-tests.sh

To fix following bug:
 # ./convert-tests.sh
     [TEST]   ext2 4k nodesize, btrfs defaults
 failed: mount /root/btrfsprogs/tests/test.img /root/btrfsprogs/tests/mnt
 # tail convert-tests-results.txt
 ...
 ############### mount /root/btrfsprogs/tests/test.img /root/btrfsprogs/tests/mnt
 mount: /root/btrfsprogs/tests/test.img is not a block device (maybe try `-o loop'?)
 failed: mount /root/btrfsprogs/tests/test.img /root/btrfsprogs/tests/mnt

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