Fix running runtests.py without arguments. (encode#7954)

Regression in aa12a5f.

Author: felixxm

runtests.py

@@ -45,5 +45,7 @@ def is_class(string):
             # `runtests.py TestCase [flags]`
             # `runtests.py test_function [flags]`
             pytest_args = ['tests', '-k', pytest_args[0]] + pytest_args[1:]
+    else:
+        pytest_args = []
 
     sys.exit(pytest.main(pytest_args))