runtime: skip TestSegv/SegvInCgo failures with "runtime: unknown pc"

This test has failed on four different builders in the past month.
Moreover, because every Go program depends on "runtime", it is likely
to be run any time a user runs 'go test all' in their own program.

Since the test is known to be flaky, let's skip it to avoid
introducing testing noise until someone has time to investigate. It
seems like we have enough samples in the builder logs to at least
start with.

For #50979

Change-Id: I9748a82fbb97d4ed95d6f474427e5aa6ecdb023d
Reviewed-on: https://go-review.googlesource.com/c/go/+/385154


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>