os/signal: increase test timeouts on ppc64*/linux

TestSignalTrace can take a bit longer to run on some
ppc64{,le} machines. I was only able to reproduce the
timeout reliably on a POWER8/ppc64le ubuntu 16.04 host.

Bump the timeout to 5 seconds.  This should be more than
sufficient for a test which occasionally takes a bit
longer to run on these builders.

Fixes #45773

Change-Id: I4177bb986561f714aabfa5b0ca44b1c90b1cd94f
Reviewed-on: https://go-review.googlesource.com/c/go/+/315049


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>