regression: CI failing on tests/async/tasync_traceback.nim

latest commit https://github.com/nim-lang/Nim/commit/87a2ced1baec059922a567e2000c0fe16cc1db52  makes CI fail:
https://travis-ci.org/nim-lang/Nim/builds/556908245
```
FAIL: tests/async/tasync_traceback.nim C
Test "tests/async/tasync_traceback.nim" in category "tests"
Failure: reExitcodesDiffer
Expected:
exitcode: 0

Gotten:
exitcode: 1

Output:
Not matched! Wrong number of lines!
```
