Simplify tests a bit

[AlekSi]

Description

No need to use //nolint:vet.

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), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3149 (2bf2e6b) into main (89de046) will decrease coverage by 11.64%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3149       +/-   ##
===========================================
- Coverage   75.09%   63.45%   -11.64%     
===========================================
  Files         393      393               
  Lines       21933    21933               
===========================================
- Hits        16470    13918     -2552     
- Misses       4493     6999     +2506     
- Partials      970     1016       +46     

see 93 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	59.13% <ø> (-12.38%)	⬇️
mongodb	5.37% <ø> (ø)
pg	48.25% <ø> (-17.04%)	⬇️
shard-1	54.76% <ø> (+0.02%)	⬆️
shard-2	24.91% <ø> (-27.92%)	⬇️
shard-3	31.52% <ø> (-26.65%)	⬇️
sqlite	35.10% <ø> (+4.85%)	⬆️
unit	24.61% <ø> (+0.15%)	⬆️

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

[rumyantseva]