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/setup/setupManualMocks.ts

@@ -91,11 +91,3 @@ window.fetch = fetchMock.sandbox();
 
 // @ts-ignore
 window.Response = Response;
-
-// set up mediaDevices mock
-Object.defineProperty(navigator, "mediaDevices", {
-    value: {
-        enumerateDevices: jest.fn().mockResolvedValue([]),
-        getUserMedia: jest.fn(),
-    },
-});