[MINOR][CORE] fix warnings on depreciated methods in...

[MINOR][CORE] fix warnings on depreciated methods in MesosClusterSchedulerSuite and DiskBlockObjectWriterSuite

## What changes were proposed in this pull request?

Fixed warnings below after scanning through warnings during build:

```
[warn] /home/jenkins/workspace/SparkPullRequestBuilder/core/src/test/scala/org/apache/spark/scheduler/cluster/mesos/MesosClusterSchedulerSuite.scala:34: imported `Utils' is permanently hidden by definition of object Utils in package mesos
[warn] import org.apache.spark.scheduler.cluster.mesos.Utils
[warn]                                                 ^
```

and
```
[warn] /home/jenkins/workspace/SparkPullRequestBuilder/core/src/test/scala/org/apache/spark/storage/DiskBlockObjectWriterSuite.scala:113: method shuffleBytesWritten in class ShuffleWriteMetrics is deprecated: use bytesWritten instead
[warn]     assert(writeMetrics.shuffleBytesWritten === file.length())
[warn]                         ^
[warn] /home/jenkins/workspace/SparkPullRequestBuilder/core/src/test/scala/org/apache/spark/storage/DiskBlockObjectWriterSuite.scala:119: method shuffleBytesWritten in class ShuffleWriteMetrics is deprecated: use bytesWritten instead
[warn]     assert(writeMetrics.shuffleBytesWritten === file.length())
[warn]                         ^
[warn] /home/jenkins/workspace/SparkPullRequestBuilder/core/src/test/scala/org/apache/spark/storage/DiskBlockObjectWriterSuite.scala:131: method shuffleBytesWritten in class ShuffleWriteMetrics is deprecated: use bytesWritten instead
[warn]     assert(writeMetrics.shuffleBytesWritten === file.length())
[warn]                         ^
[warn] /home/jenkins/workspace/SparkPullRequestBuilder/core/src/test/scala/org/apache/spark/storage/DiskBlockObjectWriterSuite.scala:135: method shuffleBytesWritten in class ShuffleWriteMetrics is deprecated: use bytesWritten instead
[warn]     assert(writeMetrics.shuffleBytesWritten === file.length())
[warn]                         ^
```

## How was this patch tested?

Tested manually on local laptop.

Author: Xin Ren <iamshrek@126.com>

Closes #14609 from keypointt/suiteWarnings.

core/src/test/scala/org/apache/spark/scheduler/cluster/mesos/MesosClusterSchedulerSuite.scala

@@ -31,7 +31,6 @@ import org.scalatest.mock.MockitoSugar
 import org.apache.spark.{LocalSparkContext, SparkConf, SparkFunSuite}
 import org.apache.spark.deploy.Command
 import org.apache.spark.deploy.mesos.MesosDriverDescription
-import org.apache.spark.scheduler.cluster.mesos.Utils
 
 class MesosClusterSchedulerSuite extends SparkFunSuite with LocalSparkContext with MockitoSugar {
 

core/src/test/scala/org/apache/spark/storage/DiskBlockObjectWriterSuite.scala

@@ -110,13 +110,13 @@ class DiskBlockObjectWriterSuite extends SparkFunSuite with BeforeAndAfterEach {
     writer.write(Long.box(20), Long.box(30))
     val firstSegment = writer.commitAndGet()
     assert(firstSegment.length === file.length())
-    assert(writeMetrics.shuffleBytesWritten === file.length())
+    assert(writeMetrics.bytesWritten === file.length())
 
     writer.write(Long.box(40), Long.box(50))
 
     writer.revertPartialWritesAndClose()
     assert(firstSegment.length === file.length())
-    assert(writeMetrics.shuffleBytesWritten === file.length())
+    assert(writeMetrics.bytesWritten === file.length())
   }
 
   test("calling revertPartialWritesAndClose() after commit() should have no effect") {
@@ -128,11 +128,11 @@ class DiskBlockObjectWriterSuite extends SparkFunSuite with BeforeAndAfterEach {
     writer.write(Long.box(20), Long.box(30))
     val firstSegment = writer.commitAndGet()
     assert(firstSegment.length === file.length())
-    assert(writeMetrics.shuffleBytesWritten === file.length())
+    assert(writeMetrics.bytesWritten === file.length())
 
     writer.revertPartialWritesAndClose()
     assert(firstSegment.length === file.length())
-    assert(writeMetrics.shuffleBytesWritten === file.length())
+    assert(writeMetrics.bytesWritten === file.length())
   }
 
   test("calling revertPartialWritesAndClose() on a closed block writer should have no effect") {