runtime: skip TestSegv/SegvInCgo earlier on ios

runtime: skip TestSegv/SegvInCgo earlier on ios

It appears to sometimes hang instead of crashing,
which can cause subsequent tests to time out.

For #59947.

Change-Id: Id4ac3c0cd5f7f345334d3e0ed3f48e40b9ff191c
Reviewed-on: https://go-review.googlesource.com/c/go/+/492075
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>