fix(frame): Fix fcc-test-frame on firefox

I used an alternative to display: none; to allow Firefox to properly run
challenge tests

Closes #10749

client/epics/frame-epic.js

@@ -35,7 +35,7 @@ const createHeader = (id = mainId) => `
 function createFrame(document, id = mainId) {
   const frame = document.createElement('iframe');
   frame.id = id;
-  frame.setAttribute('style', 'display: none');
+  frame.className = 'hide-test-frame';
   document.body.appendChild(frame);
   return frame;
 }

client/less/main.less

@@ -126,6 +126,13 @@ h1, h2, h3, h4, h5, h6, p, li {
   margin: 2px;
 }
 
+.hide-test-frame {
+  position: absolute;
+  top: -9999px;
+  left: -9999px;
+  visibility: hidden;
+}
+
 .landing-icon {
   height: 200px;
   width: 200px;