os/signal: increase wait time for signal delivery time in testcase
os/signal: increase wait time for signal delivery time in testcase

This increases the time to wait from 1 to 2 seconds in the
TestAtomicStop testcase. When running with gccgo on ppc64
& ppc64le on a loaded systems these testcases can
intermittently fail with the current value.

Updates #29046

Change-Id: If420274dd65926d933a3024903b5c757c300bd60
Reviewed-on: https://go-review.googlesource.com/c/153826
Run-TryBot: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,nawawi/go,josharian/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,josharian/go,josharian/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,akutz/go,golang/go,golang/go,CAFxX/go,akutz/go,josharian/go,cloudflare/go,golang/go,nawawi/go,akutz/go,CAFxX/go,josharian/go,josharian/go,josharian/go,cloudflare/go,golang/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,josharian/go,cloudflare/go