Clearify running testsuite as part of patch submission

[David Miller]

From: Andreas Jaeger <aj@suse.com>
Date: Tue, 17 Apr 2012 13:47:39 +0200

> Marek and myself just discussed about the testsuite on IRC and I've
> learned about the "xcheck" make target.

I have to admit having never heard of the xcheck target before
either :-/