[SPARK-26501][CORE][TEST] Fix unexpected overriden of exitFn in SparkSubmitSuite

## What changes were proposed in this pull request?

The overriden of SparkSubmit's exitFn at some previous tests in SparkSubmitSuite may cause the following tests pass even they failed when they were run separately. This PR is to fix this problem.

## How was this patch tested?

unittest

Closes #23404 from liupc/Fix-SparkSubmitSuite-exitFn.

Authored-by: Liupengcheng <liupengcheng@xiaomi.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

core/src/test/scala/org/apache/spark/deploy/SparkSubmitSuite.scala

@@ -72,27 +72,31 @@ trait TestPrematureExit {
     mainObject.printStream = printStream
 
     @volatile var exitedCleanly = false
+    val original = mainObject.exitFn
     mainObject.exitFn = (_) => exitedCleanly = true
-
-    @volatile var exception: Exception = null
-    val thread = new Thread {
-      override def run() = try {
-        mainObject.main(input)
-      } catch {
-        // Capture the exception to check whether the exception contains searchString or not
-        case e: Exception => exception = e
+    try {
+      @volatile var exception: Exception = null
+      val thread = new Thread {
+        override def run() = try {
+          mainObject.main(input)
+        } catch {
+          // Capture the exception to check whether the exception contains searchString or not
+          case e: Exception => exception = e
+        }
       }
-    }
-    thread.start()
-    thread.join()
-    if (exitedCleanly) {
-      val joined = printStream.lineBuffer.mkString("\n")
-      assert(joined.contains(searchString))
-    } else {
-      assert(exception != null)
-      if (!exception.getMessage.contains(searchString)) {
-        throw exception
+      thread.start()
+      thread.join()
+      if (exitedCleanly) {
+        val joined = printStream.lineBuffer.mkString("\n")
+        assert(joined.contains(searchString))
+      } else {
+        assert(exception != null)
+        if (!exception.getMessage.contains(searchString)) {
+          throw exception
+        }
       }
+    } finally {
+      mainObject.exitFn = original
     }
   }
 }