Fix stress tests

[AlekSi]

Description

They pass now on machines with many CPUs.

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

Merging #3502 (45e5c86) into main (a2f93bc) will increase coverage by 48.37%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main    #3502       +/-   ##
===========================================
+ Coverage   25.54%   73.92%   +48.37%     
===========================================
  Files         424      424               
  Lines       26546    26553        +7     
===========================================
+ Hits         6781    19628    +12847     
+ Misses      19104     5761    -13343     
- Partials      661     1164      +503     

Files	Coverage Δ
internal/util/testutil/teststress/stress.go	84.44% <100.00%> (+2.86%)	⬆️

... and 229 files with indirect coverage changes

Flag	Coverage Δ
filter-true	68.72% <100.00%> (+65.75%)	⬆️
hana-1	?
integration	68.72% <100.00%> (+65.75%)	⬆️
mongodb-1	4.63% <100.00%> (?)
pg-1	40.38% <100.00%> (?)
pg-2	42.98% <0.00%> (?)
pg-3	42.62% <0.00%> (?)
postgresql-1	∅ <ø> (?)
postgresql-2	∅ <ø> (?)
postgresql-3	∅ <ø> (?)
sort-false	68.72% <100.00%> (+65.75%)	⬆️
sqlite-1	39.50% <100.00%> (?)
sqlite-2	42.20% <0.00%> (?)
sqlite-3	42.13% <0.00%> (?)
unit	25.22% <100.00%> (+0.05%)	⬆️

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