GitHub Actions: Add continuous integration tests

[cclauss]

Test results: https://github.com/cclauss/cmn-tools/actions

On ubuntu-24.04-arm...
% sudo src/cmn_discover.py && src/cmn_detect_cpu.py

Writing system configuration to /home/runner/.cache/arm/cmn-system.json...
4 CPUs, 0 RN-F ports
Traceback (most recent call last):
  File "/home/runner/work/cmn-tools/cmn-tools/src/cmn_detect_cpu.py", line 386, in <module>
    prepare_system(S)
  File "/home/runner/work/cmn-tools/cmn-tools/src/cmn_detect_cpu.py", line 308, in prepare_system
    assert S.rnf_port_events
           ^^^^^^^^^^^^^^^^^
AssertionError