test: reduce flakiness of E2E rebuild test

The rebuild E2E test now waits for the development server to indicate it
has updated any clients before trying to fetch files. This improves the
reliability of the test when using the application builder.

(cherry picked from commit cd160c2)

Author: clydin

tests/legacy-cli/e2e/tests/basic/rebuild.ts

@@ -6,7 +6,7 @@ import { ngServe } from '../../utils/project';
 
 export default async function () {
   const esbuild = getGlobalVariable('argv')['esbuild'];
-  const validBundleRegEx = esbuild ? /complete\./ : /Compiled successfully\./;
+  const validBundleRegEx = esbuild ? /sent to client/ : /Compiled successfully\./;
   const lazyBundleRegEx = esbuild ? /chunk-/ : /src_app_lazy_lazy_component_ts\.js/;
 
   // Disable component stylesheet HMR to support page reload based rebuild testing.