ci: tidy up workflows

[sr-murthy]

• Rename test workflow to test.yml - the name test-python-versions-coverage.yml is unnecessarily long
• Restructure the test workflow + remove PDM in the test execution step (pip is sufficient)
• Additional (separate) build workflow (build.yml) for building package artifacts (sdist, wheel etc.), with a small test of the package installation (from the wheel) and import