test: add public api golden files

Includes a few style fixes on "* as foo" imports.

tools/public_api_guard/http/testing.d.ts

@@ -0,0 +1,19 @@
+export declare class MockBackend implements ConnectionBackend {
+    connections: any;
+    connectionsArray: MockConnection[];
+    pendingConnections: any;
+    constructor();
+    verifyNoPendingRequests(): void;
+    resolveAllConnections(): void;
+    createConnection(req: Request): MockConnection;
+}
+
+export declare class MockConnection implements Connection {
+    readyState: ReadyState;
+    request: Request;
+    response: ReplaySubject<Response>;
+    constructor(req: Request);
+    mockRespond(res: Response): void;
+    mockDownload(res: Response): void;
+    mockError(err?: Error): void;
+}