Support party_id

Send party_id on events and check the party_id of incoming events matches

Includes a basic test to assert that it actually does: we should
build out a decent test suite for calls as there's a lot of edge-case
functionality that can break and slip through the cracks (eg. glare).
This is a start.

Fixes https://github.com/matrix-org/matrix-js-sdk/issues/1511

spec/TestClient.js

@@ -69,6 +69,9 @@ export function TestClient(
 
     this.deviceKeys = null;
     this.oneTimeKeys = {};
+    this._callEventHandler = {
+        calls: new Map(),
+    };
 }
 
 TestClient.prototype.toString = function() {
@@ -230,3 +233,7 @@ TestClient.prototype.flushSync = function() {
         logger.log(`${this}: flushSync completed`);
     });
 };
+
+TestClient.prototype.isFallbackICEServerAllowed = function() {
+    return true;
+};