fix(core/testing): clean up the core testing public API (#9466)

Previously, we were exporting internal mocks and helpers. Move these
to core/testing/testing_internal or remove them if they were
never used.

Remove deprecated items - injectAsync, clearPendingTimers.

BREAKING CHANGE:

Remove the following APIs from `@angular/core/testing`, which have been deprecated or were
never intended to be publicly exported:

```
injectAsync
clearPendingTimers
Log
MockAppliacationHref
MockNgZone
clearPendingTimers
getTypeOf
instantiateType
```

Instead of `injectAsync`, use `async(inject())`.

`clearPendingTimers` is no longer required.

modules/@angular/forms/test/validators_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Log, fakeAsync, flushMicrotasks, tick} from '@angular/core/testing';
+import {fakeAsync, flushMicrotasks, tick} from '@angular/core/testing';
 import {afterEach, beforeEach, ddescribe, describe, expect, iit, it, xit} from '@angular/core/testing/testing_internal';
 import {AbstractControl, FormControl, Validators} from '@angular/forms';