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/core/testing/async.ts

@@ -24,6 +24,11 @@
 export function async(fn: Function): Function {
   return () => new Promise<void>((finishCallback, failCallback) => {
            var AsyncTestZoneSpec = (Zone as any /** TODO #9100 */)['AsyncTestZoneSpec'];
+           if (AsyncTestZoneSpec === undefined) {
+             throw new Error(
+                 'AsyncTestZoneSpec is needed for the async() test helper but could not be found. ' +
+                 'Please make sure that your environment includes zone.js/dist/async-test.js');
+           }
            var testZoneSpec = new AsyncTestZoneSpec(finishCallback, failCallback, 'test');
            var testZone = Zone.current.fork(testZoneSpec);
            return testZone.run(fn);