Unit test util/time is broken on OpenBSD

This is in master, I think we should check if it's in 0.2.8 and fix it before the stable release:

util/time: 
  FAIL src/test/test_util.c:302: assert(LONG_MAX OP_EQ tv_udiff(&start, &end)): 2147483647 vs -6005000
  [time FAILED]

https://buildbot.pixelminers.net/builders/OpenBSD/builds/980/steps/shell_3/logs/stdio