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 angular#9975
Closes angular#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';
```