runtime: skip TestSignalIgnoreSIGTRAP on known-flaky OpenBSD builders

This test is flaky, and the cause is suspected to be an OpenBSD kernel bug.
Since there is no obvious workaround on the Go side, skip the test on
builders whose versions are known to be affected.

Fixes #17496

Change-Id: Ifa70061eb429e1d949f0fa8a9e25d177afc5c488
Reviewed-on: https://go-review.googlesource.com/c/go/+/222856


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>