[SPARK-8937] [TEST] A setting spark.unsafe.exceptionOnMemoryLeak is missing in ScalaTest config.

`spark.unsafe.exceptionOnMemoryLeak` is present in the config of surefire.

```
        <!-- Surefire runs all Java tests -->
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-surefire-plugin</artifactId>
          <version>2.18.1</version>
          <!-- Note config is repeated in scalatest config -->
...

<spark.unsafe.exceptionOnMemoryLeak>true</spark.unsafe.exceptionOnMemoryLeak>
            </systemProperties>
...
```

 but is absent in the config ScalaTest.

Author: Kousuke Saruta <[email protected]>

Closes apache#7308 from sarutak/add-setting-for-memory-leak and squashes the following commits:

95644e7 [Kousuke Saruta] Added a setting for memory leak

Author: sarutak

pom.xml

@@ -1339,6 +1339,7 @@
               <spark.ui.enabled>false</spark.ui.enabled>
               <spark.ui.showConsoleProgress>false</spark.ui.showConsoleProgress>
               <spark.driver.allowMultipleContexts>true</spark.driver.allowMultipleContexts>
+              <spark.unsafe.exceptionOnMemoryLeak>true</spark.unsafe.exceptionOnMemoryLeak>
             </systemProperties>
           </configuration>
           <executions>