[travis] [async] flaky test: tests/async/t7758.nim

[timotheecour]

discard """
  file: "t7758.nim"
  exitcode: 0
"""
import asyncdispatch

proc task() {.async.} =
  await sleepAsync(1000)

when isMainModule:
  var counter = 0
  var f = task()
  while not f.finished:
    inc(counter)
    poll()

doAssert counter == 2

it happened in this PR #8682 which is (IMO) unrelated to the failure
https://travis-ci.org/nim-lang/Nim/jobs/419901855

PASS: timplicititems.nim C (10.99310398 secs)
PASS: t5888.nim C (11.55188513 secs)
FAIL: t7758.nim C
Test "tests/async/t7758.nim" in category "async"
Failure: reExitcodesDiffer
Expected:
exitcode: 0
Gotten:
exitcode: 1
Output:
t7758.nim(17) t7758
system.nim(3759) failedAssertImpl
system.nim(3752) raiseAssert
system.nim(2805) sysFatal
Error: unhandled exception: counter == 2 [AssertionError]
PASS: tcollections_to_string.nim C (24.46789384 secs)

that seems like a bug could be lurking somewhere?