macOS x86_64 fails test-timers
From https://travis-ci.org/torproject/tor/jobs/419913265
FAIL: src/test/test-timers
==========================
mean difference: 874495 usec
standard deviation: 1993793.721891 usec
Either your system is under ridiculous load, or the timer backend is broken.
FAIL src/test/test-timers (exit status: 1)