Fix ./runtests.py failing without any args

[matthewhughes934]

Description

Previously, running this script without args would result in an error:

$ ./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]

Duplicate of #7954