Increases stress test factor of TestConcurrentFileStreams

Also makes this test respect the Short flag, reducing the number of
threads by a factor of 4 and space usage by a factor of 16

Note: this test is probably unreasonable to run on the inmemory driver
without the Short flag
Merge pull request #840 from BrianBland/ng-storagedriver-tests

Increases stress test factor of TestConcurrentFileStreams