os/signal: test with a significantly longer fatal timeout

We've observed some occasional os-arch specific timeouts
in signal.TestSignalTrace(). While the main purpose of a
short timeout is to ensure the passing tests complete
quickly, the unexpected failure path can tolerate waiting
longer (the test is not intended to test how slow or
overloaded the OS is at the time it is run).

Fixes #46736
Change-Id: Ib392fc6ce485a919612784ca88ed76c30f4898e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/329502


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>