test: add tests for angular signals

[maxnowack]

I've tried to reuse stuff from the angular signals tests to write the tests for the reactivity adapter. Sadly angular doesn't export the functions required to running effects in the tests (https://github.com/angular/angular/blob/main/packages/core/test/signals/effect_util.ts).
It seems that it's necessary to include more angular stuff to setup testing properly, but I'm not sure how to do it, since I'm not very familiar with angular.

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (65a0b28) 100.00% compared to head (78a3de2) 96.33%.

❗ Current head 78a3de2 differs from pull request most recent head 138da17. Consider uploading reports for the commit 138da17 to get more accurate results

Files	Patch %	Lines
packages/plugin-angular/__tests__/reactivity.ts	0.00%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #306      +/-   ##
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- Coverage   100.00%   96.33%   -3.67%     
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  Files           35       37       +2     
  Lines          604      627      +23     
  Branches       137      137              
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  Hits           604      604              
- Misses           0       23      +23     

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