Drop test users on cleanup

[AlekSi]

Description

Removes the need to do it in every test.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1b1167e) 75.02% compared to head (1b748bd) 73.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3914      +/-   ##
==========================================
- Coverage   75.02%   73.85%   -1.18%     
==========================================
  Files         330      330              
  Lines       25602    25610       +8     
==========================================
- Hits        19208    18913     -295     
- Misses       5238     5570     +332     
+ Partials     1156     1127      -29     

Files	Coverage Δ
integration/setup/setup.go	80.21% <100.00%> (+0.44%)	⬆️
integration/setup/setup_compat.go	87.50% <100.00%> (+0.40%)	⬆️

... and 15 files with indirect coverage changes

Flag	Coverage Δ
filter-true	70.52% <100.00%> (-1.50%)	⬇️
hana-1	?
integration	70.52% <100.00%> (-1.50%)	⬇️
mongodb-1	5.24% <44.44%> (+0.01%)	⬆️
postgresql-1	51.15% <100.00%> (+0.14%)	⬆️
postgresql-2	49.65% <100.00%> (+0.04%)	⬆️
postgresql-3	51.90% <100.00%> (-0.11%)	⬇️
sqlite-1	50.26% <100.00%> (+0.13%)	⬆️
sqlite-2	48.67% <100.00%> (+0.01%)	⬆️
sqlite-3	51.07% <100.00%> (-0.09%)	⬇️
unit	30.82% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[henvic]

Thanks!

[rumyantseva]

👌