feat:(routing): ActivatedRouteStub support for firstChild/children

[cgatian]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[X] Feature request
[ ] Documentation issue or request
[ ] Support request
[ ] Other... Please describe:

Current behavior

firstChild is a property of ActivatedRoute but not ActivatedRouteStub

Expected behavior

Ability to stub out activated route children.

Minimal reproduction of the problem with instructions

For bug reports please provide the STEPS TO REPRODUCE and if possible a MINIMAL DEMO of the problem via
https://stackblitz.com or similar (you can use this template as a starting point: https://bit.ly/2zme3bj).

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

I have a routed component that inspects to see if it's firstChild was given a specific parameter.

[NetanelBasal]

You're welcome to submit a PR.