fix(core/testing): move ComponentFixture to core (#9386)

BREAKING CHANGE:

`ComponentFixture` will be moving out of `@angular/compiler/testing` to `@angular/core/testing` in
this release. For now, it is deprecated from `@angular/compiler/testing`.

modules/@angular/core/testing.ts

@@ -7,6 +7,7 @@
  */
 
 export * from './testing/async';
+export * from './testing/component_fixture';
 export * from './testing/fake_async';
 export * from './testing/test_injector';
 export * from './testing/testing';