os/signal: make TestStop resilient to initially-blocked signals

For reasons unknown, SIGUSR1 appears to be blocked at process start
for tests on the android-arm-corellium and android-arm64-corellium
builders. (This has been observed before, too: see CL 203957.)
Make the test resilient to blocked signals by always calling Notify
and waiting for potential signal delivery after sending any signal
that is not known to be unblocked.

Also remove the initial SIGWINCH signal from testCancel. The behavior
of an unhandled SIGWINCH is already tested in TestStop, so we don't
need to re-test that same case: waiting for an unhandled signal takes
a comparatively long time (because we necessarily don't know when it
has been delivered), so this redundancy makes the overall test binary
needlessly slow, especially since it is called from both TestReset and
TestIgnore.

Since each signal is always unblocked while we have a notification
channel registered for it, we don't need to modify any other tests:
TestStop and testCancel are the only functions that send signals
without a registered channel.

Fixes #38165
Updates #33174
Updates #15661

Change-Id: I215880894e954b62166024085050d34323431b63
Reviewed-on: https://go-review.googlesource.com/c/go/+/226461


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