Do not import commonerrors in tests

[AlekSi]

Description

We use values in all other tests.

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), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3081 (b76bb0f) into main (4d534d6) will decrease coverage by 3.33%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3081      +/-   ##
==========================================
- Coverage   76.66%   73.34%   -3.33%     
==========================================
  Files         388      388              
  Lines       21297    21297              
==========================================
- Hits        16328    15620     -708     
- Misses       4045     4719     +674     
- Partials      924      958      +34     

see 38 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	69.72% <ø> (-3.49%)	⬇️
mongodb	5.48% <ø> (-0.02%)	⬇️
pg	59.73% <ø> (-6.73%)	⬇️
shard-1	56.93% <ø> (-0.07%)	⬇️
shard-2	55.09% <ø> (-0.01%)	⬇️
shard-3	36.37% <ø> (-20.86%)	⬇️
sqlite	46.42% <ø> (-0.13%)	⬇️
unit	24.05% <ø> (-0.07%)	⬇️

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