fix(testing): add an explicit doAsyncPrecompilation step (#10015)

This removes the magic from the `inject` test helper that would inspect
the current zone and would only work with our `async` test helper.
Now, `inject` is always synchronous, and if you are using a module
that requires async precompilation, you're required to call
`doAsyncPrecompilation` in your tests.

This is part of the breaking changes introduced with the swap
to each test having an AppModule.

Closes #9975
Closes #9593

BREAKING CHANGE:

`TestInjector` is now renamed to `TestBed`

Before:

```js
import {TestInjector, getTestInjector} from '@angular/core/testing';
```

After:

```js
import {TestBed, getTestBed} from '@angular/core/testing';
```

modules/@angular/core/testing.ts

@@ -10,5 +10,5 @@ export * from './testing/async';
 export * from './testing/component_fixture';
 export * from './testing/fake_async';
 export * from './testing/test_component_builder';
-export * from './testing/test_injector';
+export * from './testing/test_bed';
 export * from './testing/testing';