Bug 1841896 - Port helper_fission_setResolution to regular mochitests r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D226878

gfx/layers/apz/test/mochitest/browser_test_group_fission.js

@@ -44,7 +44,6 @@ add_task(async function test_main() {
     { file: "helper_fission_large_subframe.html" },
     { file: "helper_fission_initial_displayport.html" },
     { file: "helper_fission_checkerboard_severity.html" },
-    { file: "helper_fission_setResolution.html" },
     { file: "helper_fission_inactivescroller_positionedcontent.html" },
     { file: "helper_fission_irregular_areas.html" },
     // add additional tests here

gfx/layers/apz/test/mochitest/helper_fission_setResolution.html

@@ -9,15 +9,6 @@
   <script src="apz_test_utils.js"></script>
   <script>
 
-fission_subtest_init();
-
-FissionTestHelper.startTestPromise
-  .then(waitUntilApzStable)
-  .then(loadOOPIFrame("testframe", "helper_fission_empty.html"))
-  .then(waitUntilApzStable)
-  .then(test)
-  .then(FissionTestHelper.subtestDone, FissionTestHelper.subtestFailed);
-
     async function test() {
       let iframeElement = document.getElementById("testframe");
 
@@ -46,9 +37,14 @@ async function test() {
         "The resolution value should be properly delivered into OOP iframes");
     }
 
+    waitUntilApzStable()
+      .then(test)
+      .then(subtestDone, subtestFailed);
+
   </script>
   <style>
-    body, html {
+    body,
+    html {
       margin: 0;
     }
   </style>

gfx/layers/apz/test/mochitest/test_group_fission.html

@@ -20,6 +20,7 @@
     ];
 
     var subtests = [
+      { "file": "helper_fission_setResolution.html", "prefs": prefs },
       {
         "file": "helper_fission_tap_on_zoomed.html",
         "prefs": [["apz.max_tap_time", 10000]],