KAFKA-7990: Close streams at the end in KafkaStreamsTest (#6334)

This fix is already in 2.1+ branches, but did not get into older branches.

Should be cherry-picked all the way to 0.10.2.

Reviewers: Matthias J. Sax <mjsax@apache.org>

streams/src/test/java/org/apache/kafka/streams/KafkaStreamsTest.java

@@ -342,6 +342,9 @@ public void apply(final String key, final String value) {
         } finally {
             // stop the thread so we don't interfere with other tests etc
             keepRunning.set(false);
+            if (streams != null) {
+                streams.close();
+            }
         }
     }