In PR, start build-front during test-server to improve build speed

[Pierre-Gilles]

Pull Request check-list

To ensure your Pull Request can be accepted as fast as possible, make sure to review and check all of these items:

• If your changes affects code, did your write the tests?
• Are tests passing? (npm test on both front/server)
• Is the linter passing? (npm run eslint on both front/server)
• Did you run prettier? (npm run prettier on both front/server)
• If you are adding a new features/services, did you run integration comparator? (npm run compare-translations on front)
• If your changes modify the API (REST or Node.js), did you modify the API documentation? (Documentation is based on comments in code)
• If you are adding a new features/services which needs explanation, did you modify the user documentation? See the GitHub repo and the website.
• Did you add fake requests data for the demo mode (front/src/config/demo.json) so that the demo website is working without a backend? (if needed) See https://demo.gladysassistant.com.

NOTE: these things are not required to open a PR and can be done afterwards / while the PR is open.

Description of change

Please provide a description of the change here. It's always best with screenshots, so don't hesitate to add some!

[codecov]

Codecov Report

Merging #1104 (af15939) into master (fe0d325) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1104   +/-   ##
=======================================
  Coverage   95.57%   95.57%           
=======================================
  Files         532      532           
  Lines        7168     7168           
=======================================
  Hits         6851     6851           
  Misses        317      317           

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