[MINOR][CORE][TEST] Fix afterEach() in TastSetManagerSuite and TaskSchedulerImplSuite

## What changes were proposed in this pull request?

In the `afterEach()` method of both `TastSetManagerSuite` and `TaskSchedulerImplSuite`, `super.afterEach()` shall be called at the end, because it shall stop the SparkContext.

https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/93706/testReport/org.apache.spark.scheduler/TaskSchedulerImplSuite/_It_is_not_a_test_it_is_a_sbt_testing_SuiteSelector_/
The test failure is caused by the above reason, the newly added `barrierCoordinator` required `rpcEnv` which has been stopped before `TaskSchedulerImpl` doing cleanup.

## How was this patch tested?
Existing tests.

Author: Xingbo Jiang <[email protected]>

Closes apache#21908 from jiangxb1987/afterEach.

Author: jiangxb1987

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

@@ -62,7 +62,6 @@ class TaskSchedulerImplSuite extends SparkFunSuite with LocalSparkContext with B
   }
 
   override def afterEach(): Unit = {
-    super.afterEach()
     if (taskScheduler != null) {
       taskScheduler.stop()
       taskScheduler = null
@@ -71,6 +70,7 @@ class TaskSchedulerImplSuite extends SparkFunSuite with LocalSparkContext with B
       dagScheduler.stop()
       dagScheduler = null
     }
+    super.afterEach()
   }
 
   def setupScheduler(confs: (String, String)*): TaskSchedulerImpl = {

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

@@ -178,12 +178,12 @@ class TaskSetManagerSuite extends SparkFunSuite with LocalSparkContext with Logg
   }
 
   override def afterEach(): Unit = {
-    super.afterEach()
     if (sched != null) {
       sched.dagScheduler.stop()
       sched.stop()
       sched = null
     }
+    super.afterEach()
   }