Document sociable unit tests technique

[soryy708]

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Is your feature request related to a problem? Please describe it

It's technically possible to make sociable unit tests with Automock, but it's not a first-class citizen.

Describe the solution you'd like

Documentation about how to achieve sociable unit tests (with current workarounds, or if it will become a first class citizen then with the new/changed api), the motivation and nuances...

What is the motivation / use case for changing the behavior?

const u1 = TestBed.create(Service1).compile();
const dep = u1.unitRef.get(Service3);
const u2 = TestBed.create(Service2).mock(Service1).using(u1.unit).mock(Service3).using(dep).compile();

This is non-trivial usage, and it has nuances: dependencies that are common between u1 and u2 must be explicitly made identical.