Fix missing part of dispose logic in DynamicProxyStep

Author: pimterry

src/handling/handler-step-impls.ts

@@ -251,7 +251,8 @@ export class PeerProxyStepImpl extends PeerProxyStep {
 export class DynamicProxyStepImpl extends DynamicProxyStep {
 
     async handle(connection: MockRTCConnection) {
-        await connection.proxyTrafficToExternalConnection();
+        const externalConn = await connection.proxyTrafficToExternalConnection();
+        this.externalConnections.push(externalConn);
 
         // This step keeps running indefinitely, until the connection closes
         return new Promise<void>((resolve) => connection.on('connection-closed', resolve));

src/webrtc/mockrtc-connection.ts

@@ -90,6 +90,7 @@ export class MockRTCConnection extends RTCConnection {
         }
 
         await this.proxyTrafficTo(this.externalConnection!);
+        return this.externalConnection!;
     }
 
     async proxyTrafficTo(externalConnection: RTCConnection) {