[SPARK-21950][SQL][PYTHON][TEST] pyspark.sql.tests.SQLTests2 should stop SparkContext.

## What changes were proposed in this pull request?

`pyspark.sql.tests.SQLTests2` doesn't stop newly created spark context in the test and it might affect the following tests.
This pr makes `pyspark.sql.tests.SQLTests2` stop `SparkContext`.

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <[email protected]>

Closes #19158 from ueshin/issues/SPARK-21950.

(cherry picked from commit 57bc1e9)
Signed-off-by: Takuya UESHIN <[email protected]>

Author: ueshin

python/pyspark/sql/tests.py

@@ -2252,8 +2252,12 @@ def test_sparksession_with_stopped_sparkcontext(self):
         self.sc.stop()
         sc = SparkContext('local[4]', self.sc.appName)
         spark = SparkSession.builder.getOrCreate()
-        df = spark.createDataFrame([(1, 2)], ["c", "c"])
-        df.collect()
+        try:
+            df = spark.createDataFrame([(1, 2)], ["c", "c"])
+            df.collect()
+        finally:
+            spark.stop()
+            sc.stop()
 
 
 class UDFInitializationTests(unittest.TestCase):