Add upsert tests

[wazir-ahmed]

Description

Closes #3856.

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 #3864 (853a281) into main (39aee11) will decrease coverage by 0.41%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3864      +/-   ##
==========================================
- Coverage   76.39%   75.98%   -0.41%     
==========================================
  Files         317      317              
  Lines       23933    23933              
==========================================
- Hits        18283    18185      -98     
- Misses       4596     4688      +92     
- Partials     1054     1060       +6     

see 18 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	72.26% <ø> (-0.39%)	⬇️
hana-1	?
integration	72.26% <ø> (-0.42%)	⬇️
mongodb-1	5.27% <ø> (ø)
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	52.13% <ø> (-0.07%)	⬇️
postgresql-2	50.74% <ø> (-0.10%)	⬇️
postgresql-3	53.10% <ø> (-0.09%)	⬇️
sqlite-1	51.33% <ø> (-0.15%)	⬇️
sqlite-2	49.80% <ø> (-0.09%)	⬇️
sqlite-3	52.19% <ø> (-0.03%)	⬇️
unit	30.58% <ø> (ø)

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