feat(testmessage): expose TestMessage interface

Author: kwonoj

src/message/TestMessage.ts

@@ -0,0 +1,10 @@
+import { Notification } from 'rxjs/Notification';
+
+/**
+ * Represents interface for single metadata value emitted by HotObservable<T> or ColdObservable<T>
+ *
+ */
+export interface TestMessage<T = string> {
+  frame: number;
+  notification: Notification<T>;
+}