fixes AssertSubscriber to terminate upstream if ASYNC fusion

Signed-off-by: Oleh Dokuka <shadowgun@i.ua>

Author: OlegDokuka

rsocket-core/src/test/java/io/rsocket/internal/subscriber/AssertSubscriber.java

@@ -943,6 +943,7 @@ void drain() {
       t = qs.poll();
       if (t == null) {
         if (done) {
+          qs.clear(); // clear upstream to terminated it due to the contract
           cdl.countDown();
           return;
         }