fix(testbed): rename TestInjector to TestBed - #9740

The TestBed contains an injector, but does not extend it - changing
the name to better reflect the actual class.

BREAKING CHANGE:

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

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