test: improve tests for --merge

[snitin315]

What kind of change does this PR introduce?
test

Did you add tests for your changes?
YES
If relevant, did you update the documentation?
NO
Summary
improve existing tests and add one more case for merging more than two configs.

Does this PR introduce a breaking change?
No

Other information
No

[alexander-akait]

In future please focus on coverage on tests which increase coverage, in future we migrate on snapshot tests with options, so it will be easy to catch problem, anyway thanks

[codecov]

Codecov Report

Merging #2399 (bbc67b2) into master (ad1a6b8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2399   +/-   ##
=======================================
  Coverage   71.74%   71.74%           
=======================================
  Files          47       47           
  Lines        2173     2173           
  Branches      573      573           
=======================================
  Hits         1559     1559           
  Misses        614      614           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ad1a6b8...bbc67b2. Read the comment docs.