docs(store): add 'Testing without TestBed' example

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

Does this PR introduce a breaking change?

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[x] No

[ngrxbot]

Preview docs changes for bee407d at https://previews.ngrx.io/pr2836-bee407da/

[timdeschryver]

LGTM.

Should we guide the user in how, when and why to use or not to use the TestBed?
This, in a separate PR.

[timdeschryver]

style: we (try) to import the angular dependencies first.

Suggested change

	import { MockStore, provideMockStore } from '@ngrx/store/testing';
	import { Injector } from '@angular/core';
	import { Injector } from '@angular/core';
	import { MockStore, provideMockStore } from '@ngrx/store/testing';