Add mockito as an explicit test dependency to spark-streaming

While sbt successfully compiles as it properly pulls the mockito dependency, maven builds have broken. We need this in ASAP.
tdas

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #9584 from brkyvz/fix-master.

streaming/pom.xml

@@ -93,6 +93,11 @@
       <artifactId>selenium-java</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
   <build>
     <outputDirectory>target/scala-${scala.binary.version}/classes</outputDirectory>