build: move private testing helpers outside platform-browser/testing (#61472)

These helpers are often imported by various tests throughout the
repository, but the helpers aren't exported/exposed from the public
entry-point; even though they confusingly reside in there.

This commit fixes this, and moves the helpers into
`packages/private/testing`. This is a preparation for the `ts_project`
migration where we don't want to leverage deep imports between packages.

PR Close #61472

packages/common/http/test/resource_spec.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
+import {isNode} from '@angular/private/testing';
 import {ApplicationRef, Injector, signal} from '@angular/core';
 import {TestBed} from '@angular/core/testing';
 import {