threadCount is 0 when surefire-junit47 and surefire-testng are used together

Initial discussion: https://github.com/easymock/easymock/pull/186#discussion_r85655490

Sample on: https://gist.github.com/juherr/6eb3e93e2db33979b7e90b63ddadc888

Workaround: Force `<threadCount>1</threadCount>` in surefire configuration