Fixes linter issues in testutils package

[olegbespalov]

What?

Fixes linter issues of the testutils packages.

Why?

As we agreed we'd like to reach ~0 issues in master branch.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

Part of #769

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (856a767) 73.22% compared to head (9c5145b) 73.20%.

❗ Current head 9c5145b differs from pull request most recent head 34cab33. Consider uploading reports for the commit 34cab33 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3480      +/-   ##
==========================================
- Coverage   73.22%   73.20%   -0.03%     
==========================================
  Files         267      267              
  Lines       20083    20083              
==========================================
- Hits        14706    14701       -5     
- Misses       4463     4468       +5     
  Partials      914      914              

Flag	Coverage Δ
ubuntu	73.14% <100.00%> (-0.03%)	⬇️
windows	73.06% <100.00%> (ø)

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