tests/test-runner: Allow tweaking 'center-new-windows' pref

This will allow tests to test placement algorithms.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3879>

src/tests/test-runner.c

@@ -1882,6 +1882,16 @@ test_case_do (TestCase    *test,
 
           g_assert_true (g_settings_set_int (wm, "auto-raise-delay", value));
         }
+      else if (strcmp (argv[1], "center-new-windows") == 0)
+        {
+          gboolean value;
+
+          if (!str_to_bool (argv[2], &value))
+            BAD_COMMAND("usage: %s %s [true|false]", argv[0], argv[1]);
+
+          g_assert_true (g_settings_set_boolean (mutter, "center-new-windows",
+                                                 value));
+        }
       else {
         BAD_COMMAND("Unknown preference %s", argv[1]);
       }