Move tests for tailbable cursors

[AlekSi]

Description

Still an experiment.

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 #3811 (a089041) into main (4e47318) will decrease coverage by 0.09%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3811      +/-   ##
==========================================
- Coverage   76.36%   76.27%   -0.09%     
==========================================
  Files         315      315              
  Lines       23556    23559       +3     
==========================================
- Hits        17988    17970      -18     
- Misses       4527     4549      +22     
+ Partials     1041     1040       -1     

Files	Coverage Δ
internal/handler/common/find.go	100.00% <ø> (ø)
internal/handler/msg_find.go	80.36% <0.00%> (-1.51%)	⬇️

... and 7 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.40% <0.00%> (-0.09%)	⬇️
hana-1	?
integration	72.40% <0.00%> (-0.09%)	⬇️
mongodb-1	5.19% <0.00%> (-0.01%)	⬇️
postgresql-1	52.50% <0.00%> (+0.12%)	⬆️
postgresql-2	51.25% <0.00%> (-0.05%)	⬇️
postgresql-3	52.69% <0.00%> (-0.26%)	⬇️
sqlite-1	51.60% <0.00%> (+0.16%)	⬆️
sqlite-2	50.50% <0.00%> (+0.06%)	⬆️
sqlite-3	51.80% <0.00%> (-0.16%)	⬇️
unit	30.75% <0.00%> (-0.01%)	⬇️

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