[SPARK-17316][TESTS] Fix MesosCoarseGrainedSchedulerBackendSuite

## What changes were proposed in this pull request?

The master is broken because #14882 didn't run mesos tests.

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #14902 from zsxwing/hotfix.

mesos/src/test/scala/org/apache/spark/scheduler/cluster/mesos/MesosCoarseGrainedSchedulerBackendSuite.scala

@@ -19,6 +19,7 @@ package org.apache.spark.scheduler.cluster.mesos
 
 import scala.collection.JavaConverters._
 import scala.collection.mutable.ArrayBuffer
+import scala.concurrent.Promise
 import scala.reflect.ClassTag
 
 import org.apache.mesos.{Protos, Scheduler, SchedulerDriver}
@@ -511,6 +512,7 @@ class MesosCoarseGrainedSchedulerBackendSuite extends SparkFunSuite
     when(taskScheduler.sc).thenReturn(sc)
     externalShuffleClient = mock[MesosExternalShuffleClient]
     driverEndpoint = mock[RpcEndpointRef]
+    when(driverEndpoint.ask(any())(any())).thenReturn(Promise().future)
 
     backend = createSchedulerBackend(taskScheduler, driver, externalShuffleClient, driverEndpoint)
   }