mount/mount06.c: new case to test MS_MOVE of mount

Create new test case "mount06.c" for MS_MOVE of mount.

Checking if file is visible from the new location after move mount.

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>