runtime: skip TestSignalIgnoreSIGTRAP on all OpenBSD builders

TestSignalIgnoreSIGTRAP is flaky on OpenBSD and the cause is suspected
to be a kernel bug. This test is currently only skipped on the previous
OpenBSD 6.2 and 6.4 builders for #17496. In the meantime the OpenBSD
builders were upgraded to more recent OpenBSD versions (currently 6.8
and 7.0). It seems the issue is still present in these OpenBSD versions
and there is no obvious workaround in Go. Thus, skip the flaky test on
OpenBSD in general.

Updates #17496
Updates #49725

Change-Id: I3577d287dcfaad7a81679db2e71540854fce065a
Reviewed-on: https://go-review.googlesource.com/c/go/+/367115


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>