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/structures/PipContainer-test.tsx

@@ -35,6 +35,7 @@ import {
     mkRoomCreateEvent,
     mockPlatformPeg,
     flushPromises,
+    useMockMediaDevices,
 } from "../../test-utils";
 import { MatrixClientPeg } from "../../../src/MatrixClientPeg";
 import { CallStore } from "../../../src/stores/CallStore";
@@ -84,6 +85,8 @@ describe("PipContainer", () => {
     };
 
     beforeEach(async () => {
+        useMockMediaDevices();
+
         user = userEvent.setup();
 
         stubClient();