Move mediaDevices mock out of setupManualMocks (#11413)

* Move `mediaDevices` mock out of `setupManualMocks`

This doesn't work well in test suites with multiple tests, because the
`mockReturnValue` is reset for subsequent tests.

In any case, having it mocked out automagically is *magical*. Let's make it
opt-in.

* clarify ts-ignore

test/components/views/messages/CallEvent-test.tsx

@@ -29,6 +29,7 @@ import {
     setupAsyncStoreWithClient,
     resetAsyncStoreWithClient,
     wrapInMatrixClientContext,
+    useMockMediaDevices,
 } from "../../../test-utils";
 import defaultDispatcher from "../../../../src/dispatcher/dispatcher";
 import { Action } from "../../../../src/dispatcher/actions";
@@ -41,9 +42,6 @@ import { ConnectionState } from "../../../../src/models/Call";
 const CallEvent = wrapInMatrixClientContext(UnwrappedCallEvent);
 
 describe("CallEvent", () => {
-    useMockedCalls();
-    jest.spyOn(HTMLMediaElement.prototype, "play").mockImplementation(async () => {});
-
     let client: Mocked<MatrixClient>;
     let room: Room;
     let alice: RoomMember;
@@ -55,6 +53,10 @@ describe("CallEvent", () => {
         jest.useFakeTimers();
         jest.setSystemTime(0);
 
+        useMockMediaDevices();
+        useMockedCalls();
+        jest.spyOn(HTMLMediaElement.prototype, "play").mockImplementation(async () => {});
+
         stubClient();
         client = mocked(MatrixClientPeg.safeGet());
         client.getUserId.mockReturnValue("@alice:example.org");