[SQL] Use TestSQLContext in Java tests

Sometimes tests were failing due to the creation of multiple `SparkContext`s in a single JVM.

Author: Michael Armbrust <michael@databricks.com>

Closes #4441 from marmbrus/javaTests and squashes the following commits:

657b1e0 [Michael Armbrust] [SQL] Use TestSQLContext in Java tests

sql/core/src/test/java/org/apache/spark/sql/api/java/JavaAPISuite.java

@@ -19,6 +19,7 @@ package org.apache.spark.sql.api.java;
 
 import java.io.Serializable;
 
+import org.apache.spark.sql.test.TestSQLContext$;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -37,14 +38,12 @@ public class JavaAPISuite implements Serializable {
 
   @Before
   public void setUp() {
-    sc = new JavaSparkContext("local", "JavaAPISuite");
-    sqlContext = new SQLContext(sc);
+    sqlContext = TestSQLContext$.MODULE$;
+    sc = new JavaSparkContext(sqlContext.sparkContext());
   }
 
   @After
   public void tearDown() {
-    sc.stop();
-    sc = null;
   }
 
   @SuppressWarnings("unchecked")

sql/core/src/test/java/org/apache/spark/sql/api/java/JavaApplySchemaSuite.java

@@ -22,6 +22,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
+import org.apache.spark.sql.test.TestSQLContext$;
 import org.junit.After;
 import org.junit.Assert;
 import org.junit.Before;
@@ -42,13 +43,12 @@ public class JavaApplySchemaSuite implements Serializable {
 
   @Before
   public void setUp() {
-    javaCtx = new JavaSparkContext("local", "JavaApplySchemaSuite");
-    javaSqlCtx = new SQLContext(javaCtx);
+    javaSqlCtx = TestSQLContext$.MODULE$;
+    javaCtx = new JavaSparkContext(javaSqlCtx.sparkContext());
   }
 
   @After
   public void tearDown() {
-    javaCtx.stop();
     javaCtx = null;
     javaSqlCtx = null;
   }