Don't try to run tests in parallel from tox

It doesn't work, just results in no tests being run:
https://github.com/zopefoundation/zope.testrunner/issues/5

tox.ini

@@ -7,7 +7,7 @@ commands =
 # Run unit tests first.
     zope-testrunner -u --test-path=src --auto-color --auto-progress
 # Only run functional tests if unit tests pass.
-    zope-testrunner -f -j99 --test-path=src --auto-color --auto-progress
+    zope-testrunner -f --test-path=src --auto-color --auto-progress
 deps =
     ZODB >= 4.0.0b2
     random2