Fix mysql unit tests

[adetunjii]

Description

Closes #3413

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

Attention: Patch coverage is 86.11111% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 75.54%. Comparing base (5a5ca29) to head (154c589).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4158      +/-   ##
==========================================
+ Coverage   74.69%   75.54%   +0.85%     
==========================================
  Files         353      339      -14     
  Lines       22738    22870     +132     
==========================================
+ Hits        16984    17278     +294     
+ Misses       4473     4253     -220     
- Partials     1281     1339      +58     

Files	Coverage Δ
internal/backends/mysql/insert.go	90.90% <100.00%> (+90.90%)	⬆️
internal/backends/mysql/mysql.go	80.64% <100.00%> (+80.64%)	⬆️
internal/backends/mysql/query.go	79.47% <100.00%> (+0.13%)	⬆️
internal/util/testutil/db.go	84.61% <100.00%> (ø)
internal/util/testutil/mysql.go	94.44% <100.00%> (+3.33%)	⬆️
integration/setup/listener.go	70.54% <0.00%> (-9.59%)	⬇️
integration/setup/setup_compat.go	85.55% <0.00%> (-7.63%)	⬇️
internal/backends/mysql/collection.go	46.15% <50.00%> (+43.12%)	⬆️

... and 116 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	67.34% <2.77%> (-1.12%)	⬇️
hana-1	?
integration	67.34% <2.77%> (-1.16%)	⬇️
mongodb-1	5.67% <2.77%> (+0.93%)	⬆️
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	46.52% <2.77%> (-1.72%)	⬇️
postgresql-2	49.51% <2.77%> (+0.87%)	⬆️
postgresql-3	49.77% <2.77%> (+0.56%)	⬆️
sqlite-1	45.65% <2.77%> (-1.80%)	⬇️
sqlite-2	48.75% <2.77%> (+0.92%)	⬆️
sqlite-3	48.94% <2.77%> (+0.66%)	⬆️
unit	34.31% <91.17%> (+1.74%)	⬆️

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