webcam: fix supportsMediaRecorder test

arrow functions do not have a prototype.

packages/@uppy/webcam/src/supportsMediaRecorder.test.js

@@ -2,8 +2,9 @@ const supportsMediaRecorder = require('./supportsMediaRecorder')
 
 describe('supportsMediaRecorder', () => {
   it('should return true if MediaRecorder is supported', () => {
-    global.MediaRecorder = () => {}
-    global.MediaRecorder.prototype.start = () => {}
+    global.MediaRecorder = class MediaRecorder {
+      start () {}
+    }
     expect(supportsMediaRecorder()).toEqual(true)
   })
 
@@ -11,10 +12,12 @@ describe('supportsMediaRecorder', () => {
     global.MediaRecorder = undefined
     expect(supportsMediaRecorder()).toEqual(false)
 
-    global.MediaRecorder = () => {}
+    global.MediaRecorder = class MediaRecorder {}
     expect(supportsMediaRecorder()).toEqual(false)
 
-    global.MediaRecorder.prototype.foo = () => {}
+    global.MediaRecorder = class MediaRecorder {
+      foo () {}
+    }
     expect(supportsMediaRecorder()).toEqual(false)
   })
 })