test: write pre-rendered univeral html file to test outputs dir

Write server side pre-rendered kitchen sync app to bazel TEST_UNDECLARED_OUTPUTS_DIR.
Update the DEBUG guide information to retrieve the generated `index-prerendered.html`

src/universal-app/DEBUG.md

@@ -6,5 +6,5 @@ the file will be stored in the `bazel-out` folder.
 
 You can retrieve the path to the file by either running:
 
-* `bazel test //src/universal-app:server_test --test_output=all`
-* `echo $(bazel info bazel-bin)/src/universal-app/index-prerendered.html`
+* `bazel run //src/universal-app:server_test --test_output=all`
+* `echo $(bazel info bazel-testlogs)/src/universal-app/server_test/test.outputs/index-prerendered.html`

src/universal-app/prerender.ts

@@ -3,7 +3,7 @@ import 'zone.js';
 
 import {renderModuleFactory} from '@angular/platform-server';
 import {readFileSync, writeFileSync} from 'fs';
-import {join} from 'path';
+import {dirname, join} from 'path';
 
 import {KitchenSinkRootServerModuleNgFactory} from './kitchen-sink-root.ngfactory';
 
@@ -16,10 +16,11 @@ const indexHtmlPath = require.resolve('./index.html');
 const result = renderModuleFactory(
     KitchenSinkRootServerModuleNgFactory,
     {document: readFileSync(indexHtmlPath, 'utf-8')});
+const outDir = process.env.TEST_UNDECLARED_OUTPUTS_DIR as string;
 
 result
   .then(content => {
-    const filename = join(__dirname, 'index-prerendered.html');
+    const filename = join(outDir, 'index-prerendered.html');
 
     console.log('Inspect pre-rendered page here:');
     console.log(`file://${filename}`);