Add more cursor tests

[noisersup]

Description

Closes #3739.

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 (c5e44a7) 72.50% compared to head (05009bd) 75.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3893      +/-   ##
==========================================
+ Coverage   72.50%   75.37%   +2.86%     
==========================================
  Files         312      325      +13     
  Lines       23873    24873    +1000     
==========================================
+ Hits        17309    18747    +1438     
+ Misses       5449     5003     -446     
- Partials     1115     1123       +8     

see 101 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.39% <ø> (-0.12%)	⬇️
hana-1	?
integration	72.39% <ø> (-0.12%)	⬇️
mongodb-1	5.23% <ø> (ø)
postgresql-1	50.48% <ø> (-0.49%)	⬇️
postgresql-2	51.86% <ø> (-1.39%)	⬇️
postgresql-3	53.39% <ø> (+1.21%)	⬆️
sqlite-1	49.59% <ø> (-0.67%)	⬇️
sqlite-2	50.97% <ø> (-1.33%)	⬇️
sqlite-3	52.53% <ø> (+1.22%)	⬆️
unit	31.36% <ø> (?)

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