internal-screenshot-test: Make distcheck pass

Add the output screenshot to CLEANFILES so it's properly removed on
distclean, and add the reference files and ini to EXTRA_DIST so
distcheck can find them.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Tested-by: Bryce Harrington <bryce@osg.samsung.com>