[SPARK-8910] Fix MiMa flaky due to port contention issue

Due to the way MiMa works, we currently start a `SQLContext` pretty early on. This causes us to start a `SparkUI` that attempts to bind to port 4040. Because many tests run in parallel on the Jenkins machines, this  causes port contention sometimes and fails the MiMa tests.

Note that we already disabled the SparkUI for scalatests. However, the MiMa test is run before we even have a chance to load the default scalatest settings, so we need to explicitly disable the UI ourselves.

Author: Andrew Or <andrew@databricks.com>

Closes #7300 from andrewor14/mima-flaky and squashes the following commits:

b55a547 [Andrew Or] Do not enable SparkUI during tests

sql/core/src/main/scala/org/apache/spark/sql/test/TestSQLContext.scala

@@ -26,10 +26,10 @@ import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 /** A SQLContext that can be used for local testing. */
 class LocalSQLContext
   extends SQLContext(
-    new SparkContext(
-      "local[2]",
-      "TestSQLContext",
-      new SparkConf().set("spark.sql.testkey", "true"))) {
+    new SparkContext("local[2]", "TestSQLContext", new SparkConf()
+      .set("spark.sql.testkey", "true")
+      // SPARK-8910
+      .set("spark.ui.enabled", "false"))) {
 
   override protected[sql] def createSession(): SQLSession = {
     new this.SQLSession()

sql/hive/src/main/scala/org/apache/spark/sql/hive/test/TestHive.scala

@@ -53,9 +53,10 @@ object TestHive
       "TestSQLContext",
       new SparkConf()
         .set("spark.sql.test", "")
-        .set(
-          "spark.sql.hive.metastore.barrierPrefixes",
-          "org.apache.spark.sql.hive.execution.PairSerDe")))
+        .set("spark.sql.hive.metastore.barrierPrefixes",
+          "org.apache.spark.sql.hive.execution.PairSerDe")
+        // SPARK-8910
+        .set("spark.ui.enabled", "false")))
 
 /**
  * A locally running test instance of Spark's Hive execution engine.