[#4267] Convert Objects API V1 tests to VCR

[Viicos]

Partly closes #4267

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[Viicos]

I kept test_backend.py out of scope as it has some mocks setup to fail, which would be harder to reproduce using VCR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.60%. Comparing base (9280bb2) to head (b11f979).
Report is 494 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4548   +/-   ##
=======================================
  Coverage   96.60%   96.60%           
=======================================
  Files         725      725           
  Lines       24125    24125           
  Branches     2870     2870           
=======================================
  Hits        23305    23305           
  Misses        559      559           
  Partials      261      261           

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[sergei-maertens]