[SPARK-25899][TESTS] Fix flaky CoarseGrainedSchedulerBackendSuite

## What changes were proposed in this pull request?

I saw CoarseGrainedSchedulerBackendSuite failed in my PR and finally reproduced the following error on a very busy machine:
```
sbt.ForkMain$ForkError: org.scalatest.exceptions.TestFailedDueToTimeoutException: The code passed to eventually never returned normally. Attempted 400 times over 10.009828643999999 seconds. Last failure message: ArrayBuffer("2", "0", "3") had length 3 instead of expected length 4.
```

The logs in this test shows executor 1 was not up when the test failed.
```
18/10/30 11:34:03.563 dispatcher-event-loop-12 INFO CoarseGrainedSchedulerBackend$DriverEndpoint: Registered executor NettyRpcEndpointRef(spark-client://Executor) (172.17.0.2:43656) with ID 2
18/10/30 11:34:03.593 dispatcher-event-loop-3 INFO CoarseGrainedSchedulerBackend$DriverEndpoint: Registered executor NettyRpcEndpointRef(spark-client://Executor) (172.17.0.2:43658) with ID 3
18/10/30 11:34:03.629 dispatcher-event-loop-6 INFO CoarseGrainedSchedulerBackend$DriverEndpoint: Registered executor NettyRpcEndpointRef(spark-client://Executor) (172.17.0.2:43654) with ID 0
18/10/30 11:34:03.885 pool-1-thread-1-ScalaTest-running-CoarseGrainedSchedulerBackendSuite INFO CoarseGrainedSchedulerBackendSuite:

===== FINISHED o.a.s.scheduler.CoarseGrainedSchedulerBackendSuite: 'compute max number of concurrent tasks can be launched' =====
```
And the following logs in executor 1 shows it was still doing the initialization when the timeout happened (at 18/10/30 11:34:03.885).
```
18/10/30 11:34:03.463 netty-rpc-connection-0 INFO TransportClientFactory: Successfully created connection to 54b6b6217301/172.17.0.2:33741 after 37 ms (0 ms spent in bootstraps)
18/10/30 11:34:03.959 main INFO DiskBlockManager: Created local directory at /home/jenkins/workspace/core/target/tmp/spark-383518bc-53bd-4d9c-885b-d881f03875bf/executor-61c406e4-178f-40a6-ac2c-7314ee6fb142/blockmgr-03fb84a1-eedc-4055-8743-682eb3ac5c67
18/10/30 11:34:03.993 main INFO MemoryStore: MemoryStore started with capacity 546.3 MB
```

Hence, I think our current 10 seconds is not enough on a slow Jenkins machine. This PR just increases the timeout from 10 seconds to 60 seconds to make the test more stable.

## How was this patch tested?

Jenkins

Closes apache#22910 from zsxwing/fix-flaky-test.

Authored-by: Shixiong Zhu <[email protected]>
Signed-off-by: gatorsmile <[email protected]>

Author: zsxwing

core/src/test/scala/org/apache/spark/scheduler/CoarseGrainedSchedulerBackendSuite.scala

@@ -30,6 +30,8 @@ import org.apache.spark.util.{RpcUtils, SerializableBuffer}
 class CoarseGrainedSchedulerBackendSuite extends SparkFunSuite with LocalSparkContext
     with Eventually {
 
+  private val executorUpTimeout = 60.seconds
+
   test("serialized task larger than max RPC message size") {
     val conf = new SparkConf
     conf.set("spark.rpc.message.maxSize", "1")
@@ -51,7 +53,7 @@ class CoarseGrainedSchedulerBackendSuite extends SparkFunSuite with LocalSparkCo
       .setMaster("local-cluster[4, 3, 1024]")
       .setAppName("test")
     sc = new SparkContext(conf)
-    eventually(timeout(10.seconds)) {
+    eventually(timeout(executorUpTimeout)) {
       // Ensure all executors have been launched.
       assert(sc.getExecutorIds().length == 4)
     }
@@ -64,7 +66,7 @@ class CoarseGrainedSchedulerBackendSuite extends SparkFunSuite with LocalSparkCo
       .setMaster("local-cluster[4, 3, 1024]")
       .setAppName("test")
     sc = new SparkContext(conf)
-    eventually(timeout(10.seconds)) {
+    eventually(timeout(executorUpTimeout)) {
       // Ensure all executors have been launched.
       assert(sc.getExecutorIds().length == 4)
     }
@@ -96,7 +98,7 @@ class CoarseGrainedSchedulerBackendSuite extends SparkFunSuite with LocalSparkCo
 
     try {
       sc.addSparkListener(listener)
-      eventually(timeout(10.seconds)) {
+      eventually(timeout(executorUpTimeout)) {
         // Ensure all executors have been launched.
         assert(sc.getExecutorIds().length == 4)
       }