Add tests for backends contract

[AlekSi]

Description

Test that updates are no-op for non-existing databases or collections.

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 #3456 (cda6ceb) into main (1182de2) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3456      +/-   ##
==========================================
+ Coverage   73.97%   74.15%   +0.18%     
==========================================
  Files         422      422              
  Lines       26375    26374       -1     
==========================================
+ Hits        19510    19558      +48     
+ Misses       5716     5661      -55     
- Partials     1149     1155       +6     

Files	Coverage Δ
internal/handlers/sqlite/msg_createindexes.go	89.53% <100.00%> (-0.03%)	⬇️

... and 16 files with indirect coverage changes

Flag	Coverage Δ
filter-true	69.15% <100.00%> (-0.09%)	⬇️
hana-1	?
integration	69.15% <100.00%> (-0.09%)	⬇️
mongodb-1	4.63% <0.00%> (+<0.01%)	⬆️
pg-1	40.58% <0.00%> (-0.03%)	⬇️
pg-2	43.24% <0.00%> (+0.09%)	⬆️
pg-3	42.90% <0.00%> (+<0.01%)	⬆️
postgresql-1	∅ <ø> (∅)
postgresql-2	?
postgresql-3	?
sort-false	69.15% <100.00%> (-0.09%)	⬇️
sqlite-1	39.74% <100.00%> (+0.05%)	⬆️
sqlite-2	42.37% <100.00%> (-0.10%)	⬇️
sqlite-3	42.39% <100.00%> (-0.01%)	⬇️
unit	25.06% <0.00%> (+0.61%)	⬆️

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[rumyantseva]

👌

[chilagrow]

Thank you for this 🙏

[noisersup]

Looks good to me, apart from failing test