ovl: update documentation for unionmount-testsuite

David's tree is no longer maintained, so point to my maintained fork.

Add --verify flag to the run example, which enables all latest features
and provides test coverage for constant st_ino/st_dev.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>