chore(test-bed): add fromInjector constructor to TestBed

Some clients do not generate static factories for test code, and instead
directly depend on TestBed constructor. This commit adds a new
constructor is basically the injector 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 run for test code), we can change the constructor
dependencies in a non-breaking way.