[SPARK-33173][CORE][TESTS] Use eventually to check numOnTaskFailed in PluginContainerSuite

### What changes were proposed in this pull request?

This PR aims to use `eventually` to fix the flakiness of the test case `SPARK-33088: executor failed tasks trigger plugin calls`.

### Why are the changes needed?

The test case checks like the following.
```scala
assert(TestSparkPlugin.executorPlugin.numOnTaskStart == 2)
assert(TestSparkPlugin.executorPlugin.numOnTaskSucceeded == 0)
assert(TestSparkPlugin.executorPlugin.numOnTaskFailed == 2)
```

Although first and second passed, the third can fail.
- https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test%20(Dashboard)/job/spark-master-test-maven-hadoop-3.2-hive-2.3-jdk-11/lastCompletedBuild/testReport/org.apache.spark.internal.plugin/PluginContainerSuite/SPARK_33088__executor_failed_tasks_trigger_plugin_calls/
- https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/129919/testReport/
```
sbt.ForkMain$ForkError: org.scalatest.exceptions.TestFailedException: 1 did not equal 2
	at org.scalatest.Assertions.newAssertionFailedException(Assertions.scala:472)
	at org.scalatest.Assertions.newAssertionFailedException$(Assertions.scala:471)
	at org.scalatest.Assertions$.newAssertionFailedException(Assertions.scala:1231)
	at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:1295)
	at org.apache.spark.internal.plugin.PluginContainerSuite.$anonfun$new$8(PluginContainerSuite.scala:161)
```

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

This only improves the robustness.

Closes #30072 from dongjoon-hyun/SPARK-33173.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

core/src/test/scala/org/apache/spark/internal/plugin/PluginContainerSuite.scala

@@ -156,9 +156,11 @@ class PluginContainerSuite extends SparkFunSuite with BeforeAndAfterEach with Lo
       case t: Throwable => // ignore exception
     }
 
-    assert(TestSparkPlugin.executorPlugin.numOnTaskStart == 2)
-    assert(TestSparkPlugin.executorPlugin.numOnTaskSucceeded == 0)
-    assert(TestSparkPlugin.executorPlugin.numOnTaskFailed == 2)
+    eventually(timeout(10.seconds), interval(100.millis)) {
+      assert(TestSparkPlugin.executorPlugin.numOnTaskStart == 2)
+      assert(TestSparkPlugin.executorPlugin.numOnTaskSucceeded == 0)
+      assert(TestSparkPlugin.executorPlugin.numOnTaskFailed == 2)
+    }
   }
 
   test("plugin initialization in non-local mode") {