Fix test race introduced by 22dffdf (java-native-access#267)

Motivation:

22dffdf added a testcase which had a race.

Modifications:

Remove the isDone() check as it is racy

Result:

Fix test-failure which can happen due the race

src/test/java/io/netty/incubator/codec/quic/QuicWritableTest.java

@@ -248,7 +248,6 @@ public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
                             clientErrorRef.set(cause);
                         }
                     }).get();
-            assertFalse(writePromise.isDone());
 
             // Let's trigger the reads. This will ensure we will consume the data and the remote peer
             // should be notified that it can write more data.