utility to wait until a value becomes truthy in a test

src/mocks/poll.js

@@ -0,0 +1,27 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+export async function poll(fn) {
+    let result;
+    do {
+        const result = fn();
+        if (result) {
+            return result;
+        } else {
+            await new Promise(setImmediate);
+        }
+    } while (1);
+}