Make test helpers harder to misuse

[AlekSi]

Refs #481.

[codecov]

Codecov Report

Merging #530 (49b98e3) into main (18aa27e) will increase coverage by 0.20%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##             main     #530      +/-   ##
==========================================
+ Coverage   63.59%   63.79%   +0.20%     
==========================================
  Files         116      116              
  Lines        6513     6522       +9     
==========================================
+ Hits         4142     4161      +19     
+ Misses       1828     1823       -5     
+ Partials      543      538       -5     

Impacted Files	Coverage Δ
internal/util/testutil/equal.go	57.39% <76.47%> (+8.64%)	⬆️

Flag	Coverage Δ
FerretDB	52.14% <70.58%> (+0.31%)	⬆️
MongoDB	6.62% <70.58%> (+0.37%)	⬆️
integration	52.17% <70.58%> (+0.31%)	⬆️
unit	40.39% <76.47%> (+0.24%)	⬆️

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