Test for asserted identity

This is out first CallHandler test(!) Switches react-sdk to use
createCall on the client object so we can stub this out in the test.
Add a bunch more stubs to the test client.

There's more stuff in this test that has scope to be used more
widely, like waiting for a certain dispatch and mocking out rooms
with particular sets of users in them: we could consider moving these
out to test utils if we wanted.

src/utils/DMRoomMap.ts

@@ -55,6 +55,15 @@ export default class DMRoomMap {
         return DMRoomMap.sharedInstance;
     }
 
+    /**
+     * Set the shared instance to the instance supplied
+     * Used by tests
+     * @param inst the new shared instance
+     */
+    public static setShared(inst: DMRoomMap) {
+        DMRoomMap.sharedInstance = inst;
+    }
+
     /**
      * Returns a shared instance of the class
      * that uses the singleton matrix client