Self test on parsos files to check if there's no syntax errors.

test/test_file_python_errors.py

@@ -0,0 +1,23 @@
+import os
+
+import parso
+
+
+def get_python_files(path):
+    for dir_path, dir_names, file_names in os.walk(path):
+        for file_name in file_names:
+            if file_name.endswith('.py'):
+                yield os.path.join(dir_path, file_name)
+
+
+def test_parso_files(each_version):
+    """
+    There are obviously no syntax erros in the Python code of parso. However
+    parso should output the same for all versions.
+    """
+    grammar = parso.load_grammar(version=each_version)
+    path = os.path.dirname(os.path.dirname(__file__)) + '/parso'
+    for file in get_python_files(path):
+        tree = grammar.parse(path=file)
+        errors = list(tree._iter_errors(grammar))
+        assert not errors