Make aggregate stats integration tests pass for SQLite

[noisersup]

Description

• aggregate_documents_test.go
• aggregate_stats_compat_test.go

Closes #3131.

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 #3151 (9aa937d) into main (957a563) will decrease coverage by 0.17%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3151      +/-   ##
==========================================
- Coverage   44.32%   44.16%   -0.17%     
==========================================
  Files         393      393              
  Lines       21873    21875       +2     
==========================================
- Hits         9696     9661      -35     
- Misses      11373    11420      +47     
+ Partials      804      794      -10     

Files Changed	Coverage Δ
integration/setup/test_helpers.go	93.33% <0.00%> (+0.47%)	⬆️

... and 35 files with indirect coverage changes

Flag	Coverage Δ
integration	34.56% <0.00%> (-0.15%)	⬇️
mongodb	5.38% <0.00%> (-0.01%)	⬇️
shard-1	31.74% <0.00%> (-0.37%)	⬇️
shard-2	24.86% <0.00%> (+1.21%)	⬆️
shard-3	27.29% <0.00%> (-1.15%)	⬇️
sqlite	34.11% <0.00%> (-0.30%)	⬇️
unit	24.68% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

LGTM overall, one tiny question.

[chilagrow]

The failing CI tests were due to flakiness of sqlite. so it's good to go

[rumyantseva]