test/README: Change suggested screen size

The dash-state test needs a surface with a width of 1500 pixels. If the screen
size is smaller than that, the boilerplate backends that create a window on the
X server can't properly do their job because part of the window would be outside
of the screen. This means people should use a screen large enough for all the
needed test surfaces. 1680 seemed like a more-or-less realistic value here.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

test/README

@@ -128,7 +128,7 @@ Here are some of the relevant details:
     X server that uses only software for all rendering. One such X
     server is Xvfb which can be started like this:
 
-	Xvfb -screen 0 1280x1024x24 -ac -nolisten tcp :2
+	Xvfb -screen 0 1680x1024x24 -ac -nolisten tcp :2
 
     after which the test suite can be run against it like so: