[SPARK-28101][DSTREAM][TEST] Fix Flaky Test: InputStreamsSuite.Modified files are correctly detected in JDK9+

## What changes were proposed in this pull request?

It seems that https://bugs.openjdk.java.net/browse/JDK-8068730 makes `InputStreamsSuite` very flaky.

<img width="903" alt="error" src="https://user-images.githubusercontent.com/9700541/59727067-017eb780-91e9-11e9-8bb0-ac5f4c1bc44d.png">

As we can see the Jenkins result, this can be reproduced frequently with JDK9+.
```
$ build/sbt "streaming/testOnly *.InputStreamsSuite"
[info] - Modified files are correctly detected. *** FAILED *** (134 milliseconds)
[info]   Set("renamed") did not equal Set() (InputStreamsSuite.scala:312)
[info]   org.scalatest.exceptions.TestFailedException:
```

The reason is the `renamed.txt`'s modification time becomes greater than the clock in JDK9+ and Spark ignored it with **not selected** message. In JDK8,  the modification time generated by this test case doesn't have `milliseconds` part.
```
Getting new files for time 1560896662000, ignoring files older than 1560896659679
file:/.../streaming/subdir/renamed.txt not selected as mod time 1560896662679 > current time 1560896662000
file:/.../streaming/subdir/existing ignored as mod time 1560896657679 <= ignore time 1560896659679
Finding new files took 0 ms
New files at time 1560896662000 ms:
```

## How was this patch tested?

Pass the Jenkins and manually repeat the following with JDK11 10 times.
```
$ build/sbt "streaming/testOnly *.InputStreamsSuite"
```

Closes apache#24904 from dongjoon-hyun/SPARK-28101.

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

Author: dongjoon-hyun

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

@@ -293,8 +293,7 @@ class InputStreamsSuite extends TestSuiteBase with BeforeAndAfter {
         val textPath = new Path(generatedSubDir, "renamed.txt")
         write(textPath, "renamed\n")
         val now = clock.getTimeMillis()
-        val modTime = now + durationMs / 2
-        fs.setTimes(textPath, modTime, modTime)
+        fs.setTimes(textPath, now, now)
         val textFilestatus = fs.getFileStatus(existingFile)
         assert(textFilestatus.getModificationTime < now + durationMs)