testscripts/test_fs_bind.sh: adapt to system which marks mount point as "shared" defaultly

Linux kernel does not allow to move a mount residing under a
shared mount.

This patch tries to make sure test cases of "move" run under
a private mount.

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>