Rename testing flag

[AlekSi]

Description

For consistency with a future flag.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• 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), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2437 (c226771) into main (0a3a45c) will increase coverage by 0.19%.
The diff coverage is 14.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2437      +/-   ##
==========================================
+ Coverage   26.71%   26.91%   +0.19%     
==========================================
  Files         393      393              
  Lines       19289    19291       +2     
==========================================
+ Hits         5154     5192      +38     
+ Misses      13581    13545      -36     
  Partials      554      554              

Impacted Files	Coverage Δ
integration/setup/listener.go	0.00% <0.00%> (ø)
integration/setup/setup.go	73.72% <ø> (ø)
integration/setup/test_helpers.go	81.48% <0.00%> (ø)
internal/handlers/pg/msg_count.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_delete.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_distinct.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_explain.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_find.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_findandmodify.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_update.go	0.00% <0.00%> (ø)
... and 12 more

... and 6 files with indirect coverage changes

Flag	Coverage Δ
integration	4.71% <0.00%> (-0.01%)	⬇️
mongodb	4.71% <0.00%> (-0.01%)	⬇️
unit	25.57% <16.66%> (+0.20%)	⬆️

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