Add an E2E test suite

e2e_tests/test_basic.py

@@ -0,0 +1,13 @@
+#!/usr/bin/env python3
+
+from helpers import *
+
+
+def test_invalid_command():
+    cmd = grm(["whatever"], is_invalid=True)
+    assert "USAGE" in cmd.stderr
+
+
+def test_help():
+    cmd = grm(["--help"])
+    assert "USAGE" in cmd.stdout