os/signal: in TestStop, skip the final "unexpected signal" check for SIGUSR1 on Android

In CL 226138, I updated TestStop to have more uniform behavior for its signals.
However, that test seems to always fail for SIGUSR1 on the Android ARM builders.

I'm not sure what's special about Android for this particular case,
but let's skip the test to unbreak the builders while I investigate.

For #38165
Updates #33174

Change-Id: I35a70346cd9757a92acd505a020bf95e6871405c
Reviewed-on: https://go-review.googlesource.com/c/go/+/226458


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>