os/signal: run TestNotifyContextNotifications subtests in parallel
os/signal: run TestNotifyContextNotifications subtests in parallel

If we run out of time on the first subtest, we don't want to start the
second one with essentially no time remaining. (Moreover, there is no
compelling reason not to run these tests in parallel, since they send
signals to separate processes.)

For #51054.

Change-Id: I0424e08c3a9d2db986568d5a5c004859b52f7c51
Reviewed-on: https://go-review.googlesource.com/c/go/+/398454
Trust: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Henrique Vicente de Oliveira Pinto <d390f26e2f50ad5716a9c69c58de1f5df9730e3b@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,nawawi/go