tests: fix the cursor race in internal-screenshot

tests: fix the cursor race in internal-screenshot

This fix also depends on "compositor-headless: do not create a seat".

If we lose the race against weston-desktop-shell setting cursors, which
is very rare, we get a cursor image in the screenshot, causing the test
to fail. This is now fixed by moving the (remaining) cursor out of the
way.

Arguably we should have better solutions for this, but that is another
story. This is a stop-gap measure we can copy also in new
screenshooting tests.

v2: Remove the example code for how to trigger the race, and rewrite the
big comment.

Cc: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>