test(spread): make ros linter test less strict

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

---

This spread test is now less strict but still verifies that:

1. libpng library warning is filtered
2. ros2-humble library warnings are not filtered

Fixes #4256
(CRAFT-1872)

[codecov-commenter]

Codecov Report

Merging #4369 (1ae07ec) into main (b357ef5) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff             @@
##             main    #4369      +/-   ##
==========================================
- Coverage   89.11%   89.09%   -0.03%     
==========================================
  Files         318      317       -1     
  Lines       21165    21116      -49     
==========================================
- Hits        18862    18814      -48     
+ Misses       2303     2302       -1     

see 4 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more