rate: skip TestLongRunningQPS on OpenBSD

time.Sleep is significantly oversleeping on OpenBSD, rendering this
test invalid.  Skip until that's resolved.

Updates #14183.

Change-Id: Ib0b79d198a0600454c5f74383d9933a2c05ddd2f
Reviewed-on: https://go-review.googlesource.com/19171
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>