[SPARK-16722][TESTS] Fix a StreamingContext leak in StreamingContextSuite when eventually fails

## What changes were proposed in this pull request?

This PR moves `ssc.stop()` into `finally` for `StreamingContextSuite.createValidCheckpoint` to avoid leaking a StreamingContext since leaking a StreamingContext will fail a lot of tests and make us hard to find the real failure one.

## How was this patch tested?

Jenkins unit tests

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #14354 from zsxwing/ssc-leak.

streaming/src/test/scala/org/apache/spark/streaming/StreamingContextSuite.scala

@@ -819,10 +819,13 @@ class StreamingContextSuite extends SparkFunSuite with BeforeAndAfter with Timeo
     ssc.checkpoint(checkpointDirectory)
     ssc.textFileStream(testDirectory).foreachRDD { rdd => rdd.count() }
     ssc.start()
-    eventually(timeout(10000 millis)) {
-      assert(Checkpoint.getCheckpointFiles(checkpointDirectory).size > 1)
+    try {
+      eventually(timeout(30000 millis)) {
+        assert(Checkpoint.getCheckpointFiles(checkpointDirectory).size > 1)
+      }
+    } finally {
+      ssc.stop()
     }
-    ssc.stop()
     checkpointDirectory
   }