docs(testing): expose how to define the type of an signal store insta…

[distante]

…nce.

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Please check if your PR fulfills the following requirements:

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

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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
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What is the current behavior?

There is no information of how to get the correct type of a Signal Store instance for testing.

What is the new behavior?

Information was added.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

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[markostanimirovic]

This is already mentioned on the "FAQ" page (How can I get the type of a SignalStore): https://ngrx.io/guide/signals/faq

[rainerhahnekamp]

@markostanimirovic. Yes but we should at least refer to it in the testing section. this PR came out of a discussion Reddit.