chore(test-bed): Add fromInjector constructor to TestBed.

Some clients do not generate static factories for test code, thus
depending on TestBed constructor. The new constructor is basically the
factory method, requiring only injector vs a full set of parameters.

Once clients have been moved to use fromInjector method (or even better
a transformer for test code), we can change the constructor dependencies
in a non-braking way.