refactor(core): re-export private exports from testing (angular#55101)

This commit re-exports private exports  from testing package.

PR Close angular#55101

Author: atscott

packages/core/testing/public_api.ts

@@ -14,5 +14,6 @@
  * Entry point for all public APIs of this package.
  */
 export * from './src/testing';
+export * from './private_export';
 
 // This file only reexports content of the `src` folder. Keep it that way.