Add test to verify no-warnings config option fails when not boolean

tests/test_config.py

@@ -38,6 +38,11 @@ class SimpleConfigTestCase(unittest.TestCase):
         new = config.YamlLintConfig('no-warnings: true')
         self.assertTrue(new.no_warnings)
 
+        with self.assertRaisesRegex(
+                config.YamlLintConfigError,
+                'invalid config: no-warnings should be a bool'):
+            config.YamlLintConfig('no-warnings: foobar')
+
     def test_parse_config(self):
         new = config.YamlLintConfig('rules:\n'
                                     '  colons:\n'