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/platform-browser-dynamic/test/testing_public_browser_spec.ts

@@ -7,7 +7,7 @@
  */
 
 import {it, iit, xit, describe, ddescribe, xdescribe, expect, beforeEach, beforeEachProviders, inject,} from '@angular/core/testing';
-import {async, fakeAsync, flushMicrotasks, Log, tick,} from '@angular/core/testing';
+import {async, fakeAsync, flushMicrotasks, tick,} from '@angular/core/testing';
 
 import {ROUTER_DIRECTIVES, Route} from '@angular/router-deprecated';