testcases/lib: Add inject_randr15_outputinfo argument

Allow tests to specify a file name for inject_randr15's
--getoutputinfo_reply command-line parameter.

testcases/lib/i3test.pm.in

@@ -864,6 +864,7 @@ sub launch_with_config {
         dont_create_temp_dir => $args{dont_create_temp_dir},
         validate_config => $args{validate_config},
         inject_randr15 => $args{inject_randr15},
+        inject_randr15_outputinfo => $args{inject_randr15_outputinfo},
     );
 
     # If we called i3 with -C, we wait for it to exit and then return as