xfstests: Assume yes when test device is not partitioned

xfstests: Assume yes when test device is not partitioned

If an entire device used for file system creation, the mkfs.ext2/3/4
asks for confirmation before proceeding. Since the device is
configured for testing it is safe to assume positive response
during mkfs.

The patch also replaces hard coded mkfs path with MKFS_PROG variable.

Signed-off-by: Chaitanya Kulkarni <chaitanyakulkarni15@gmail.com>
Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>