[SPARK-28115][CORE][TEST] Fix flaky test: SparkContextSuite.test resource scheduling under local-cluster mode

## What changes were proposed in this pull request?

The test `SparkContextSuite.test resource scheduling under local-cluster mode` has been flaky, because it expects the size of `sc.statusTracker.getExecutorInfos` be the same as the number of executors, while the returned list contains both the driver and executors.

## How was this patch tested?

Updated existing tests.

Closes apache#24917 from jiangxb1987/getExecutorInfos.

Authored-by: Xingbo Jiang <[email protected]>
Signed-off-by: Xiangrui Meng <[email protected]>

Author: jiangxb1987

core/src/main/scala/org/apache/spark/SparkStatusTracker.scala

@@ -99,6 +99,7 @@ class SparkStatusTracker private[spark] (sc: SparkContext, store: AppStatusStore
   /**
    * Returns information of all known executors, including host, port, cacheSize, numRunningTasks
    * and memory metrics.
+   * Note this include information for both the driver and executors.
    */
   def getExecutorInfos: Array[SparkExecutorInfo] = {
     store.executorList(true).map { exec =>

core/src/test/scala/org/apache/spark/SparkContextSuite.scala

@@ -750,9 +750,7 @@ class SparkContextSuite extends SparkFunSuite with LocalSparkContext with Eventu
       sc = new SparkContext(conf)
 
       // Ensure all executors has started
-      eventually(timeout(10.seconds)) {
-        assert(sc.statusTracker.getExecutorInfos.size == 1)
-      }
+      TestUtils.waitUntilExecutorsUp(sc, 1, 10000)
       assert(sc.resources.size === 1)
       assert(sc.resources.get(GPU).get.addresses === Array("5", "6"))
       assert(sc.resources.get(GPU).get.name === "gpu")
@@ -780,9 +778,7 @@ class SparkContextSuite extends SparkFunSuite with LocalSparkContext with Eventu
       sc = new SparkContext(conf)
 
       // Ensure all executors has started
-      eventually(timeout(10.seconds)) {
-        assert(sc.statusTracker.getExecutorInfos.size == 1)
-      }
+      TestUtils.waitUntilExecutorsUp(sc, 1, 10000)
       // driver gpu resources file should take precedence over the script
       assert(sc.resources.size === 1)
       assert(sc.resources.get(GPU).get.addresses === Array("0", "1", "8"))
@@ -855,9 +851,7 @@ class SparkContextSuite extends SparkFunSuite with LocalSparkContext with Eventu
       sc = new SparkContext(conf)
 
       // Ensure all executors has started
-      eventually(timeout(60.seconds)) {
-        assert(sc.statusTracker.getExecutorInfos.size == 3)
-      }
+      TestUtils.waitUntilExecutorsUp(sc, 3, 60000)
 
       val rdd = sc.makeRDD(1 to 10, 9).mapPartitions { it =>
         val context = TaskContext.get()