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