Make a test reliable by waiting for font loads.

Differential Revision: https://phabricator.services.mozilla.com/D205543

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1887522
gecko-commit: f5a00c2272483b66f764456af1f4dd06e548837c
gecko-reviewers: layout-reviewers, jfkthame

css/css-scoping/font-face-006.html

@@ -16,7 +16,7 @@
 </style>
 <div id="host"></div>
 <script>
-test(function() {
+promise_test(async function() {
   host.attachShadow({ mode: "open" }).innerHTML = `
     <style>
       :host::before, :host::after {
@@ -27,6 +27,8 @@
     <slot></slot>
   `;
 
+  await document.fonts.ready;
+
   //shrinkwrapped size for a default font will be a bit more than 80-90
   //if the font is applied, it will be a bit more than 160
   assert_greater_than(document.getElementById('host').offsetWidth, 160);