Make AssertEqual helper handle duplicate keys

[AlekSi]

Description

*types.Document.Get panics in that case.

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: 3 lines in your changes are missing coverage. Please review.

Comparison is base (af36e33) 74.92% compared to head (8b97cb8) 68.11%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3911      +/-   ##
==========================================
- Coverage   74.92%   68.11%   -6.81%     
==========================================
  Files         330      330              
  Lines       25595    25598       +3     
==========================================
- Hits        19177    17437    -1740     
- Misses       5257     6952    +1695     
- Partials     1161     1209      +48     

Files	Coverage Δ
internal/util/testutil/equal.go	50.90% <70.00%> (-0.70%)	⬇️

... and 73 files with indirect coverage changes

Flag	Coverage Δ
filter-true	64.06% <60.00%> (-7.85%)	⬇️
hana-1	?
integration	64.06% <60.00%> (-7.85%)	⬇️
mongodb-1	5.22% <60.00%> (+0.14%)	⬆️
postgresql-1	50.99% <60.00%> (-0.01%)	⬇️
postgresql-2	49.65% <60.00%> (+0.02%)	⬆️
postgresql-3	?
sqlite-1	50.17% <60.00%> (-0.06%)	⬇️
sqlite-2	48.65% <60.00%> (-0.01%)	⬇️
sqlite-3	?
unit	30.83% <70.00%> (+<0.01%)	⬆️

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[rumyantseva]

👌