Add simple tests using pytest & CI using github actions

in case your EDITOR is a command + flags, you have to split it properly before
feeding it into `subprocess.Popen()`, otherwise it will fail to launch

shlex.split EDITOR environment variable

The mock_find_executable fixture was only mocking one import path based on
Python version, but get_editor() imports find_executable at runtime with a
try/except fallback pattern. This caused test failures in rpmbuild environments
where the mock didn't target the actually used function.

Updated the fixture to mock both distutils.spawn.find_executable and
shutil.which, then return the appropriate mock based on what's available at
runtime.

fix: resolve test failures in package build