tests: Accept async condition functions for `helper.waitFor()`

src/tests/frontend/helper.js

@@ -211,9 +211,9 @@ const helper = {};
       return _fail(...args);
     };
 
-    const check = () => {
+    const check = async () => {
       try {
-        if (!conditionFunc()) return;
+        if (!await conditionFunc()) return;
         deferred.resolve();
       } catch (err) {
         deferred.reject(err);

src/tests/frontend/specs/helper.js

@@ -210,6 +210,19 @@ describe('the test helper', function () {
         await helper.waitFor(() => true, 0);
       });
     });
+
+    it('accepts async functions', async function () {
+      await helper.waitFor(async () => true).fail(() => {});
+      // Make sure it checks the truthiness of the Promise's resolved value, not the truthiness of
+      // the Promise itself (a Promise is always truthy).
+      let ok = false;
+      try {
+        await helper.waitFor(async () => false, 0).fail(() => {});
+      } catch (err) {
+        ok = true;
+      }
+      expect(ok).to.be(true);
+    });
   });
 
   describe('the waitForPromise method', function () {