cmd/go: fix longtest builder after runtime/internal/atomic
cmd/go: fix longtest builder after runtime/internal/atomic

Loosen test to accept staleness in any runtime/internal package.
Fixes longtest builder.

Change-Id: Iffda9c1d1816f76c782aecc4d53c0ea3e6580587
Reviewed-on: https://go-review.googlesource.com/c/go/+/443755
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Auto-Submit: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go