[SPARK-15508][STREAMING][TESTS] Fix flaky test: JavaKafkaStreamSuite.testKafkaStream

## What changes were proposed in this pull request?

`JavaKafkaStreamSuite.testKafkaStream` assumes when `sent.size == result.size`, the contents of `sent` and `result` should be same. However, that's not true. The content of `result` may not be the final content.

This PR modified the test to always retry the assertions even if the contents of `sent` and `result` are not same.

Here is the failure in Jenkins: http://spark-tests.appspot.com/tests/org.apache.spark.streaming.kafka.JavaKafkaStreamSuite/testKafkaStream

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #13281 from zsxwing/flaky-kafka-test.

external/kafka-0-8/src/test/java/org/apache/spark/streaming/kafka/JavaKafkaStreamSuite.java

@@ -122,14 +122,23 @@ public class JavaKafkaStreamSuite implements Serializable {
     ssc.start();
 
     long startTime = System.currentTimeMillis();
-    boolean sizeMatches = false;
-    while (!sizeMatches && System.currentTimeMillis() - startTime < 20000) {
-      sizeMatches = sent.size() == result.size();
+    AssertionError lastError = null;
+    while (System.currentTimeMillis() - startTime < 20000) {
+      try {
+        Assert.assertEquals(sent.size(), result.size());
+        for (Map.Entry<String, Integer> e : sent.entrySet()) {
+          Assert.assertEquals(e.getValue().intValue(), result.get(e.getKey()).intValue());
+        }
+        return;
+      } catch (AssertionError e) {
+        lastError = e;
+      }
       Thread.sleep(200);
     }
-    Assert.assertEquals(sent.size(), result.size());
-    for (Map.Entry<String, Integer> e : sent.entrySet()) {
-      Assert.assertEquals(e.getValue().intValue(), result.get(e.getKey()).intValue());
+    if (lastError != null) {
+      throw lastError;
+    } else {
+      Assert.fail("timeout");
     }
   }
 }