fixed very slightly flaky tests. Now uses float timestamps and float del...

...tas instead of round-to-integer to test for simultaneity.
Testing for simultaneity by converting timestamps to integers, and then asserting for equality almost always worked. But once in a while, the two timestamps, while only a few msecs apart, fall in different seconds (e.g. result[0] = 34,998 and result[1] = 35,000). If this happens, the test failed although for all practical purposes, the two timestamps were at the same time. (This happened in https://travis-ci.org/capistrano/sshkit/jobs/33774970)