APP_INITIALIZER contract not honoured with TestBed

## I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)

[x] Bug report  

[ ] Performance issue

[ ] Feature request

[ ] Documentation issue or request

[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

[ ] Other... Please describe:

## Current behavior

When importing a Module providing a promise-based APP_INITIALIZER into a TestBed, the tests are executed before the promise is fully resolved.

There was an issue for this very problem (#16204) but the reporter closed because he found a workaround.

## Expected behavior

The promise of an APP_INITIALIZER should be fully resolved, exactly as when running the application that uses this module normally.

## Minimal reproduction of the problem with instructions

Hadn't had the time, but I expect that it should be simple to know if is already implemented or not to behave like expected. If yes, I will produce a repro.

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

Same contract with tests or real app.

## Environment

Angular version: 6.0.7

Start by examining the TestBed and APP_INITIALIZER entry points and reproduce the reported promise-based initialization behavior with a minimal test. Done means tests wait for the APP_INITIALIZER promise to resolve before executing, matching the normal application contract.

Written by the indexing model from the issue text.