SPARK-3177 (on Master Branch)

The JIRA and PR was original created for branch-1.1, and move to master branch now. 
Chester

The Issue is due to that yarn-alpha and yarn have different APIs for certain class fields. In this particular case,  the ClientBase using reflection to to address this issue, and we need to different way to test the ClientBase's method.  Original ClientBaseSuite using getFieldValue() method to do this. But it doesn't work for yarn-alpha as the API returns an array of String instead of just String (which is the case for Yarn-stable API).

 To fix the test, I add a new method

  def getFieldValue2[A: ClassTag, A1: ClassTag, B](clazz: Class[_], field: String,
                                                      defaults: => B)
                              (mapTo:  A => B)(mapTo1: A1 => B) : B =
    Try(clazz.getField(field)).map(_.get(null)).map {
      case v: A => mapTo(v)
      case v1: A1 => mapTo1(v1)
      case _ => defaults
    }.toOption.getOrElse(defaults)

to handle the cases where the field type can be either type A or A1. In this new method the type A or A1 is pattern matched and corresponding mapTo function (mapTo or mapTo1) is used.
without ClassTag, you will get the following warnings:
 
 abstract type pattern A is unchecked since it is eliminated by erasure
 abstract type pattern A1 is unchecked since it is eliminated by erasure

so that 
case v: A => mapTo(v)
case v1: A1 => mapTo1(v1)

will not match

with ClassTag, the runtime type are returned and above match will work as expected