Test 4 -- Ring Stress-test

Communication occurs around multiple rings. For each ring, each rank is assigned a left and right neighbor to send/recv from. We keep multiple rings going at the same time, but they're all independent of each other.

This test generates lots of traffic but not any performance data, as there's no good measure for how much traffic should be expected to perform well. Instead the result of this test is simply a binary pass or fail.

Output lines should appear every few seconds, and the total runtime should be in roughly the 12 - 16 minute range.