list: concurrency test sometimes fails

[tomhughes]

It's not consistent, but the "list: concurrency" test in stream-list.test.js sometimes fails:

# list: concurrency
not ok 243 concurrency 10
  ---
    operator: deepEqual
    expected: |-
      { done: 30, ops: 40, skipped: 14, total: 77 }
    actual: |-
      { done: 20, ops: 30, skipped: 9, total: 77 }
    at: null._onTimeout (/builddir/build/BUILD/tilelive-5.11.0/test/stream-list.test.js:105:11)
  ...

I assume it's something to do with the precise timing, given the test seems to be looking at how much work is done, given rate limited streams, in a given amount of time.