test: Use project .npmrc for tests

[Artur-]

Makes it possible to use a custom registry for dependency testing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.57%. Comparing base (242f830) to head (91de092).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3089   +/-   ##
=======================================
  Coverage   92.57%   92.57%           
=======================================
  Files          85       85           
  Lines        3164     3164           
  Branches      775      775           
=======================================
  Hits         2929     2929           
  Misses        183      183           
  Partials       52       52           

Flag	Coverage Δ
unittests	92.57% <ø> (ø)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

Hi @Artur- and @Artur-, when i performed cherry-pick to this commit to 24.6, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 5719651
error: could not apply 5719651... test: Use project .npmrc for tests (#3089)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".