Bug 1871385: Set pref to enable popover in related a11y test. r=ayeddi

Differential Revision: https://phabricator.services.mozilla.com/D197126

accessible/tests/browser/e10s/browser_caching_states.js

@@ -493,6 +493,10 @@ addAccessibleTask(
   <div id="mypopover" popover>Popover content</div>
   `,
   async function (browser, docAcc) {
+    await SpecialPowers.pushPrefEnv({
+      set: [["dom.element.popover.enabled", true]],
+    });
+
     const show = findAccessibleChildByID(docAcc, "show-popover-btn");
     const hide = findAccessibleChildByID(docAcc, "hide-popover-btn");
     testStates(show, STATE_COLLAPSED, 0);