Fix flaky ByteStreamMessageOutputStreamTest.writeAfterStreamClosed() (

#5427)

Motivation:

`ByteStreamMessageOutputStreamTest.writeAfterStreamClosed()` tests if
`ByteStreamMessageOutputStream` raises an IOException after the
enclosing `ByteStreamMessage` is aborted.

If a `StreamMessage` has been subscribed by another `Subscriber`, the
abortion process may happen in a different thread asynchronously.

Modifications:

- Wait until the abortion of `ByteStreamMessage` completes.

Results:

Closes #4553

core/src/test/java/com/linecorp/armeria/common/stream/ByteStreamMessageOutputStreamTest.java

@@ -24,6 +24,7 @@
 import java.io.Closeable;
 import java.io.IOException;
 import java.time.Duration;
+import java.util.concurrent.CompletionException;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -175,6 +176,12 @@ void writeAfterStreamClosed() throws InterruptedException {
         StepVerifier.create(byteStreamMessage, 2)
                     .expectNext(httpData(0), httpData(1))
                     .then(byteStreamMessage::abort)
+                    .then(() -> {
+                        // Wait for the abortion to be completed.
+                        assertThatThrownBy(() -> byteStreamMessage.whenComplete().join())
+                                .isInstanceOf(CompletionException.class)
+                                .hasCauseInstanceOf(AbortedStreamException.class);
+                    })
                     .then(wait::countDown)
                     .verifyError(AbortedStreamException.class);
         end.await();