[TEST] Fix for benchmark tests

- Fix bug where repeatedly calling computeSummaryStatistics() could
  accumulate some values incorrectly
- Fix check for number of responsive nodes on list is <= number of
  candidate benchmark nodes
- Add public getters for summary statistics
- Add javadoc for new getters
- Add javadoc comments about API use
- Improve abort and status tests by calling awaitBusy() to wait for jobs
  to be completely submitted before testing them