test: add more test cases for avoid-ignoring-return-values rule

[dkrutskikh]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 353

• Total lines of source code: 6256

• Total classes: 237

• Average Cyclomatic Number per line of code: 0.35 / 1

• Average Source Lines of Code per method: 6

• Total tech debt: 1034.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #628 (f1e72ae) into master (9b79547) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #628   +/-   ##
=======================================
  Coverage   86.03%   86.03%           
=======================================
  Files         232      232           
  Lines        4955     4955           
=======================================
  Hits         4263     4263           
  Misses        692      692           

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