[cmake] Fix coverage

[hiergaut]

Pull Request Desription

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UPDATE the form below to describe your PR

Please check if the PR fulfills these requirements

• The commit message follows our guidelines (see CONTRIBUTING)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Be aware that the PR request cannot be accepted if it doesn't pass the Continuous Integration tests.

• What kind of change does this PR introduce?

  • bug fix
  • feature
  • docs update
  • other:

• What is the current behavior? (You can also link to an open issue here)
  No check of decline coverage of tests performed.

• What is the new behavior (if this is a feature change)?
  Check decline (compare to stats on main branch) for each branch.
  Save performance tests stats only for main branch.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  If you add code or new file, be aware to add tests for features you suggest.

• Other information:

[hiergaut]

Ready for review.

[hiergaut]

All source (cpp) file have to be a corresponding test. I'll do this in the future. Issue #51 reported.