Fix hang in Channel subscribe/close/drain

When we polled a Channel, we forgot to re-insert the CLOSE or FAIL
message. This meant that under certain use patterns, such as when we
had a subscription to a Channel, closed it, and then tried to drain it
with a `for` loop, we could get a hang, since the draining loop never
saw the CLOSE message.

src/core/Channel.pm6

@@ -78,12 +78,14 @@ my class Channel does Awaitable {
             nqp::istype(msg, CHANNEL_CLOSE),
             nqp::stmts(
               $!closed_promise_vow.keep(Nil),
+              nqp::push($!queue, msg),
               Nil
             ),
             nqp::if(
               nqp::istype(msg, CHANNEL_FAIL),
               nqp::stmts(
                 $!closed_promise_vow.break(msg.error),
+                nqp::push($!queue, msg),
                 Nil
               ),
               msg

t/spectest.data

@@ -836,6 +836,7 @@ S16-io/words.t
 S16-unfiled/rebindstdhandles.t
 S17-channel/basic.t
 S17-channel/stress.t     # stress slow
+S17-channel/subscription-drain-in-react.t
 S17-lowlevel/atomic.t    # moar
 S17-lowlevel/atomic-ops.t # moar
 S17-lowlevel/cas.t