Improve randomString mock for better snapshots (#11375)

test/setupTests.ts

@@ -0,0 +1,48 @@
+/*
+Copyright 2022 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.
+*/
+
+import "@testing-library/jest-dom";
+import "blob-polyfill";
+import { randomString } from "matrix-js-sdk/src/randomstring";
+import { mocked } from "jest-mock";
+
+import { PredictableRandom } from "./predictableRandom"; // https://github.com/jsdom/jsdom/issues/2555
+
+// Fake random strings to give a predictable snapshot for IDs
+jest.mock("matrix-js-sdk/src/randomstring");
+beforeEach(() => {
+    const chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
+    const mockRandom = new PredictableRandom();
+    mocked(randomString).mockImplementation((len) => {
+        let ret = "";
+        for (let i = 0; i < len; ++i) {
+            const v = mockRandom.get() * chars.length;
+            const m = ((v % chars.length) + chars.length) % chars.length; // account for negative modulo
+            ret += chars.charAt(Math.floor(m));
+        }
+        return ret;
+    });
+});
+
+// Very carefully enable the mocks for everything else in
+// a specific order. We use this order to ensure we properly
+// establish an application state that actually works.
+//
+// These are also require() calls to make sure they get called
+// synchronously.
+require("./setup/setupManualMocks"); // must be first
+require("./setup/setupLanguage");
+require("./setup/setupConfig");