ci: do not check coverage of tests dir

[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?

---

pytest was checking the coverage of the tests dir.

For example:

[codecov-commenter]

Codecov Report

Merging #4222 (79bee0c) into main (ba7d74c) will decrease coverage by 4.93%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main    #4222      +/-   ##
==========================================
- Coverage   93.88%   88.95%   -4.93%     
==========================================
  Files         569      296     -273     
  Lines       43730    20157   -23573     
==========================================
- Hits        41054    17931   -23123     
+ Misses       2676     2226     -450     

see 291 files with indirect coverage changes

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