test: fix tests so it fails when servePath is not working

This is due that when targeting an html file, when it doesn't exists it redirects to the root page. So instead use a `js` files to test

Author: alan-agius4

packages/angular_devkit/build_angular/test/dev-server/serve-path_spec_large.ts

@@ -17,16 +17,13 @@ describe('Dev Server Builder serve path', () => {
   beforeEach(done => host.initialize().toPromise().then(done, done.fail));
   afterEach(done => host.restore().toPromise().then(done, done.fail));
 
-  // TODO: review this test, it seems to pass with or without the servePath.
   it('works', (done) => {
     const overrides: Partial<DevServerBuilderOptions> = { servePath: 'test/' };
 
     runTargetSpec(host, devServerTargetSpec, overrides).pipe(
       tap((buildEvent) => expect(buildEvent.success).toBe(true)),
-      concatMap(() => from(request('http://localhost:4200/test/'))),
-      tap(response => expect(response).toContain('<title>HelloWorldApp</title>')),
-      concatMap(() => from(request('http://localhost:4200/test/abc/'))),
-      tap(response => expect(response).toContain('<title>HelloWorldApp</title>')),
+      concatMap(() => from(request('http://localhost:4200/test/polyfills.js'))),
+      tap(response => expect(response).toContain('window["webpackJsonp"]')),
       take(1),
     ).toPromise().then(done, done.fail);
   }, 30000);