Implement fake-outputs option (cmdline, cfg) for multi-monitor testing

This kills the dependency on xdmx and makes the testsuite simpler
and more flexible (in the output sizes / configurations).

testcases/t/501-scratchpad.t

@@ -5,7 +5,15 @@
 # ticket #596, bug present until up to commit
 # 89dded044b4fffe78f9d70778748fabb7ac533e9.
 #
-use i3test;
+use i3test i3_autostart => 0;
+
+my $config = <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+fake-outputs 1024x768+0+0,1024x768+1024+0
+EOT
+my $pid = launch_with_config($config);
 
 my $i3 = i3(get_socket_path());
 
@@ -92,4 +100,6 @@ $second = fresh_workspace(output => 0);
 
 verify_scratchpad_switch($first, $second);
 
+exit_gracefully($pid);
+
 done_testing;