[YARN] [MINOR] Avoid hard code port number in YarnShuffleService test

Current port number is fixed as default (7337) in test, this will introduce port contention exception, better to change to a random number in unit test.

squito , seems you're author of this unit test, mind taking a look at this fix? Thanks a lot.

```
[info] - executor state kept across NM restart *** FAILED *** (597 milliseconds)
[info]   org.apache.hadoop.service.ServiceStateException: java.net.BindException: Address already in use
[info]   at org.apache.hadoop.service.ServiceStateException.convert(ServiceStateException.java:59)
[info]   at org.apache.hadoop.service.AbstractService.init(AbstractService.java:172)
[info]   at org.apache.spark.network.yarn.YarnShuffleServiceSuite$$anonfun$1.apply$mcV$sp(YarnShuffleServiceSuite.scala:72)
[info]   at org.apache.spark.network.yarn.YarnShuffleServiceSuite$$anonfun$1.apply(YarnShuffleServiceSuite.scala:70)
[info]   at org.apache.spark.network.yarn.YarnShuffleServiceSuite$$anonfun$1.apply(YarnShuffleServiceSuite.scala:70)
[info]   at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)
[info]   at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
[info]   at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
[info]   at org.scalatest.Transformer.apply(Transformer.scala:22)
[info]   at org.scalatest.Transformer.apply(Transformer.scala:20)
[info]   at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:166)
[info]   at org.apache.spark.SparkFunSuite.withFixture(SparkFunSuite.scala:42)
...
```

Author: jerryshao <sshao@hortonworks.com>

Closes #8502 from jerryshao/avoid-hardcode-port.

yarn/src/test/scala/org/apache/spark/network/yarn/YarnShuffleServiceSuite.scala

@@ -37,6 +37,7 @@ class YarnShuffleServiceSuite extends SparkFunSuite with Matchers with BeforeAnd
     yarnConfig.set(YarnConfiguration.NM_AUX_SERVICES, "spark_shuffle")
     yarnConfig.set(YarnConfiguration.NM_AUX_SERVICE_FMT.format("spark_shuffle"),
       classOf[YarnShuffleService].getCanonicalName)
+    yarnConfig.setInt("spark.shuffle.service.port", 0)
 
     yarnConfig.get("yarn.nodemanager.local-dirs").split(",").foreach { dir =>
       val d = new File(dir)