Ensure compared times are within a second of one another instead of
rounding, which can flake if the two times are different enough to
do round to different values.
Tested using the golang.org/x/tools/cmd/stress tool.
The following set of commands fail without this patch:
$ go get golang.org/x/tools/cmd/stress
$ go test -o server.test github.com/coreos/dex/server
$ stress ./server.test -test.run=TestOAuth2CodeFlow
219 runs so far, 0 failures
425 runs so far, 0 failures
618 runs so far, 0 failures
802 runs so far, 0 failures
^C
Closes #699