[MINOR][TEST][STREAMING] make "testDir" able to be claened after test.

It's a minor bug in test case. `val testDir = null` will keep be `null` as it's immutable, so in finally block, nothing will be cleaned. Another `testDir` variable created in try block is only visible in try block.

## How was this patch tested?
Run existing test case and passed.

Author: mwws <wei.mao@intel.com>

Closes #12999 from mwws/SPARK_MINOR.

streaming/src/test/scala/org/apache/spark/streaming/InputStreamsSuite.scala

@@ -140,10 +140,10 @@ class InputStreamsSuite extends TestSuiteBase with BeforeAndAfter {
   }
 
   test("binary records stream") {
-    val testDir: File = null
+    var testDir: File = null
     try {
       val batchDuration = Seconds(2)
-      val testDir = Utils.createTempDir()
+      testDir = Utils.createTempDir()
       // Create a file that exists before the StreamingContext is created:
       val existingFile = new File(testDir, "0")
       Files.write("0\n", existingFile, StandardCharsets.UTF_8)
@@ -363,10 +363,10 @@ class InputStreamsSuite extends TestSuiteBase with BeforeAndAfter {
   }
 
   def testFileStream(newFilesOnly: Boolean) {
-    val testDir: File = null
+    var testDir: File = null
     try {
       val batchDuration = Seconds(2)
-      val testDir = Utils.createTempDir()
+      testDir = Utils.createTempDir()
       // Create a file that exists before the StreamingContext is created:
       val existingFile = new File(testDir, "0")
       Files.write("0\n", existingFile, StandardCharsets.UTF_8)