os/signal: remove some arbitrary timeouts in tests
This should fix the test flake found in
https://build.golang.org/log/
48ffb18e85dda480b7a67e8305dd03ee8337f170.
For #58901.
Change-Id: I1fcdd713a78e6b7c81e38133ce5f42f7f448a1a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/541115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>