[SPARK-20957][SS][TESTS] Fix o.a.s.sql.streaming.StreamingQueryManagerSuite listing

## What changes were proposed in this pull request?

When stopping StreamingQuery, StreamExecution will set `streamDeathCause` then notify StreamingQueryManager to remove this query. So it's possible that when `q2.exception.isDefined` returns `true`, StreamingQueryManager's active list still has `q2`.

This PR just puts the checks into `eventually` to fix the flaky test.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18180 from zsxwing/SPARK-20957.

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQueryManagerSuite.scala

@@ -78,9 +78,9 @@ class StreamingQueryManagerSuite extends StreamTest with BeforeAndAfter {
       eventually(Timeout(streamingTimeout)) {
         require(!q2.isActive)
         require(q2.exception.isDefined)
+        assert(spark.streams.get(q2.id) === null)
+        assert(spark.streams.active.toSet === Set(q3))
       }
-      assert(spark.streams.get(q2.id) === null)
-      assert(spark.streams.active.toSet === Set(q3))
     }
   }