ls was not found on GH runner. use a different program to test stderr

tests/test_executable.py

@@ -65,19 +65,21 @@ def test_stderr(tmp_path):
     """
     Tests the `stderr` attribute of the `Executable` class
     """
-    cmd = which("ls", required=True)
+    cmd = which("which", required=True)
 
     stdout_file = tmp_path / 'stdout'
     stderr_file = tmp_path / 'stderr'
     with pytest.raises(ProcessError):
-        cmd("--help", output=str(stdout_file), error=str(stderr_file))
+        cmd("-h", output=str(stdout_file), error=str(stderr_file))
 
     # Assert there is no stdout
     with open(str(stdout_file)) as fh:
         assert fh.read() == ''
 
-    # Assert stderr is not empty, '--help' is an unrecognized option
+    # Assert stderr is not empty, '-h' is a bad option for `which`
     with open(str(stderr_file)) as fh:
         stderr = fh.read()
         assert stderr != ''
-        print(stderr)
+        # Depending on the OS, the error message may vary
+        # This was seen on macOS
+        # assert stderr == "/usr/bin/which: illegal option -- h" + '\n' + "usage: which [-as] program ..." + '\n'