Require runnable dispatcher subcommands (#1585)

* Require runnable dispatcher subcommands

* Use modern dispatcher test loader

tests/test_odysseus_dispatcher.py

@@ -0,0 +1,24 @@
+import importlib.machinery
+import importlib.util
+from pathlib import Path
+
+
+def _load_dispatcher():
+    path = Path(__file__).resolve().parent.parent / "scripts" / "odysseus"
+    loader = importlib.machinery.SourceFileLoader("odysseus_dispatcher_under_test", str(path))
+    spec = importlib.util.spec_from_loader(loader.name, loader)
+    module = importlib.util.module_from_spec(spec)
+    loader.exec_module(module)
+    return module
+
+
+def test_is_runnable_subcommand_requires_executable_file(tmp_path):
+    cli = _load_dispatcher()
+    sub = tmp_path / "odysseus-demo"
+    sub.write_text("#!/bin/sh\n")
+    sub.chmod(0o644)
+
+    assert cli._is_runnable_subcommand(sub) is False
+
+    sub.chmod(0o755)
+    assert cli._is_runnable_subcommand(sub) is True