Add unit tests for hangup / reject (#2606)

* Add unit tests for hangup / reject

Fixes https://github.com/vector-im/element-call/issues/537

* Fix some bugs where we carried on with the call after it had been ended

spec/test-utils/webrtc.ts

@@ -109,6 +109,12 @@ export class MockRTCPeerConnection {
             sdp: DUMMY_SDP,
         });
     }
+    createAnswer() {
+        return Promise.resolve({
+            type: 'answer',
+            sdp: DUMMY_SDP,
+        });
+    }
     setRemoteDescription() {
         return Promise.resolve();
     }