Add proper testing to CI + some modification to release procedure.

[sergey-yaroslavtsev]

1. add proper testing using add env variable to start pymca in test mode for CI #1147 instead just running the app
2. do not download from testPyPI use only local wheels (to avoid crash when last version is not ready for one of OS-arch-cpy) - maybe less logical but more stable
3. use only version from local file - not from testPyPI (sometimes version was read incorrectly leading to crash at last steps)
4. download wheels-artifacts at once using pattern as suggested by @t20100
5. add tests after creating DMG - maybe overkill, but then i would rather delete it for merged app, as DMG is an end product
6. remove TEST_APP_FUNCTION in _MacOS.yml since test function is now only two lines
7. if tests fail then CI should fail - https://github.com/sergey-yaroslavtsev/pymca/actions/runs/19329081104/job/55287584638 due to exit $LASTEXITCODE and set -e
8. add .src.tgz to the release assets - so now assets should completely mimic SourceForge
9. navigation image moved
10. small additions to CONTRIBUTION.rst

Notes:

1. testing of these modifications was done in https://github.com/sergey-yaroslavtsev/pymca/actions/runs/19329081104/job/55287584638 (last fail is due to missing DMG file but all modified steps have passed)
2. modification of CI is complicated since uploading and signing parts should be commented - hard to do full test outside of main repo - hopefully significant modifications will not be required in nearest future
3. the "easiest" way I found to modify CI is to create a new fork and manually comment all secrets/uploads/signing then test - then uncomment...

If you think commit history is "dirty" I can redo it from fresh branch.

[sergey-yaroslavtsev]

@mretegan can you please check _MacOS.yml file. Especially what happens after packaging-signing since i was not able to test it outside of main repo.