clean up variable naming: threadNumber becomes forkNumber
[SUREFIRE-949] update documentation with new forkCount / reuseForks options

- also make ${surefire.threadNumber} replacement undocumented, as its naming is totally misleading and might give users wrong impressions
make test more robust by removing some assertions that depend on current system load
[SUREFIRE-949] update documentation with new forkCount / reuseForks options