docs(signals): add testing guide

[rainerhahnekamp]

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PR Checklist

The formatted version is available at: https://github.com/rainerhahnekamp/ngrx/blob/docs/signals/testing/projects/ngrx.io/content/guide/signals/signal-store/testing.md.

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] 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:

Closes #4206

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[jdegand]

Missing backtick on line 56 around TestBed. Some inconsistency in using TestBed and TestBed. Line 62 is missing the backticks.

Could add the links to ng-mocks and other libraries mentioned.

[rainerhahnekamp]

@markostanimirovic, @timdeschryver, @jdegand, @markgoho

I've incorporated all your suggestions and left them open for your convenience. I'm now ready for the next review round and promise it will be faster as the first one. 😄

[brandonroberts]

The formatting on some of the code snippets is a little off, but I think in general its ready to be merged, and we can follow up with additional PRs with more cleanup.

[timdeschryver]

@markostanimirovic, @timdeschryver, @jdegand, @markgoho

I've incorporated all your suggestions and left them open for your convenience. I'm now ready for the next review round and promise it will be faster as the first one. 😄

Thanks @rainerhahnekamp 🙏 I will go through this later this week (probably over the weekend)

[rainerhahnekamp]

Thanks @timdeschryver