test: add cli argument testing and fix docstrings

tests/test_config_flag.py

@@ -1,16 +1,28 @@
-"""Check that the --config command-line flag works"""
-
+"""Test user option overrides using --config and load_options"""
 from proselint.tools import load_options
 
 
 def test_load_options_function():
+    """Test load_options by specifying a user options path"""
     overrides = load_options("tests/test_config_flag_proselintrc")
     assert load_options()["checks"]["uncomparables.misc"]
     assert not overrides["checks"]["uncomparables.misc"]
 
 
 def test_load_fallbacks():
+    """Test load_options with a fallback path"""
     fallbacks = load_options(None, ["tests/test_config_flag_proselintrc"])
     assert not fallbacks["checks"]["uncomparables.misc"]
     fallbacks = load_options(None, ["./.proselintrc"])
     assert fallbacks["checks"]["uncomparables.misc"]
+
+
+def test_config_flag():
+    """Test the --config CLI argument"""
+    output = subprocess.run(["python", "-m", "proselint", "--demo"],
+                            stdout=subprocess.PIPE, encoding='utf-8')
+    assert "uncomparables.misc" in output.stdout
+    output = subprocess.run(["python", "-m", "proselint", "--demo", "--config",
+                             "tests/test_config_flag_proselintrc"],
+                            stdout=subprocess.PIPE, encoding='utf-8')
+    assert "uncomparables.misc" not in output.stdout