[SPARK-29104][CORE][TESTS] Fix PipedRDDSuite to use eventually to check thread termination

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

`PipedRDD` will invoke `stdinWriterThread.interrupt()` at task completion, and `obj.wait` will get `InterruptedException`. However, there exists a possibility which the thread termination gets delayed because the thread starts from `obj.wait()` with that exception. To prevent test flakiness, we need to use `eventually`. Also, This PR fixes the typo in code comment and variable name.

### Why are the changes needed?

```
- stdin writer thread should be exited when task is finished *** FAILED ***
  Some(Thread[stdin writer for List(cat),5,]) was not empty (PipedRDDSuite.scala:107)
```

- https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test%20(Dashboard)/job/spark-master-test-maven-hadoop-2.7/6867/testReport/junit/org.apache.spark.rdd/PipedRDDSuite/stdin_writer_thread_should_be_exited_when_task_is_finished/

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

No.

### How was this patch tested?

Manual.

We can reproduce the same failure like Jenkins if we catch `InterruptedException` and sleep longer than the `eventually` timeout inside the test code. The following is the example to reproduce it.
```scala
val nums = sc.makeRDD(Array(1, 2, 3, 4), 1).map { x =>
  try {
    obj.synchronized {
      obj.wait() // make the thread waits here.
    }
  } catch {
    case ie: InterruptedException =>
      Thread.sleep(15000)
      throw ie
  }
  x
}
```

Closes #25808 from dongjoon-hyun/SPARK-29104.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: dongjoon-hyun

core/src/test/scala/org/apache/spark/rdd/PipedRDDSuite.scala

@@ -21,16 +21,18 @@ import java.io.File
 
 import scala.collection.JavaConverters._
 import scala.collection.Map
+import scala.concurrent.duration._
 import scala.io.Codec
 
 import org.apache.hadoop.fs.Path
 import org.apache.hadoop.io.{LongWritable, Text}
 import org.apache.hadoop.mapred.{FileSplit, JobConf, TextInputFormat}
+import org.scalatest.concurrent.Eventually
 
 import org.apache.spark._
 import org.apache.spark.util.Utils
 
-class PipedRDDSuite extends SparkFunSuite with SharedSparkContext {
+class PipedRDDSuite extends SparkFunSuite with SharedSparkContext with Eventually {
   val envCommand = if (Utils.isWindows) {
     "cmd.exe /C set"
   } else {
@@ -100,11 +102,16 @@ class PipedRDDSuite extends SparkFunSuite with SharedSparkContext {
 
     assert(result.collect().length === 0)
 
-    // collect stderr writer threads
-    val stderrWriterThread = Thread.getAllStackTraces.keySet().asScala
-      .find { _.getName.startsWith(PipedRDD.STDIN_WRITER_THREAD_PREFIX) }
-
-    assert(stderrWriterThread.isEmpty)
+    // SPARK-29104 PipedRDD will invoke `stdinWriterThread.interrupt()` at task completion,
+    // and `obj.wait` will get InterruptedException. However, there exists a possibility
+    // which the thread termination gets delayed because the thread starts from `obj.wait()`
+    // with that exception. To prevent test flakiness, we need to use `eventually`.
+    eventually(timeout(10.seconds), interval(1.second)) {
+      // collect stdin writer threads
+      val stdinWriterThread = Thread.getAllStackTraces.keySet().asScala
+        .find { _.getName.startsWith(PipedRDD.STDIN_WRITER_THREAD_PREFIX) }
+      assert(stdinWriterThread.isEmpty)
+    }
   }
 
   test("advanced pipe") {