Very flaky tests

Hi,

I am one of the maintainers of Serf in Debian. Until recently, we were not running tests, but since I re-enabled them, we are getting a high number of failures in different architectures: https://buildd.debian.org/status/logs.php?pkg=golang-github-hashicorp-serf&ver=0.8.1%2Bgit20171021.c20a0b1%7Eds1-3

Locally, the tests seem to run fine. But as soon as I create high load on my laptop, I can reproduce the flakiness. Evidently, many tests rely on timing that is not enough on loaded machines.

Thanks.