[HUDI-568] Improve unit test coverage

Classes improved:
* HoodieTableMetaClient
* RocksDBDAO
* HoodieRealtimeFileSplit

hudi-hadoop-mr/pom.xml

@@ -101,6 +101,12 @@
       <artifactId>junit</artifactId>
       <scope>test</scope>
     </dependency>
+
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-all</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>