Fix running runtests.py without arguments.

[felixxm]

Regression in aa12a5f.

$ ./runtests.py 
Traceback (most recent call last):
  File "./runtests.py", line 49, in <module>
    sys.exit(pytest.main(pytest_args))
NameError: name 'pytest_args' is not defined

[carltongibson]

Thanks @felixxm