[SPARK-25606][TEST] Reduce DateExpressionsSuite test time costs in Jenkins

## What changes were proposed in this pull request?

Reduce `DateExpressionsSuite.Hour` test time costs in Jenkins by reduce iteration times.

## How was this patch tested?
Manual tests on my local machine.
before:
```
- Hour (34 seconds, 54 milliseconds)
```
after:
```
- Hour (2 seconds, 697 milliseconds)
```

Closes apache#22632 from wangyum/SPARK-25606.

Authored-by: Yuming Wang <[email protected]>
Signed-off-by: gatorsmile <[email protected]>

Author: wangyum

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/DateExpressionsSuite.scala

@@ -273,9 +273,9 @@ class DateExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     for (tz <- Seq(TimeZoneGMT, TimeZonePST, TimeZoneJST)) {
       val timeZoneId = Option(tz.getID)
       c.setTimeZone(tz)
-      (0 to 24).foreach { h =>
-        (0 to 60 by 15).foreach { m =>
-          (0 to 60 by 15).foreach { s =>
+      (0 to 24 by 6).foreach { h =>
+        (0 to 60 by 30).foreach { m =>
+          (0 to 60 by 30).foreach { s =>
             c.set(2015, 18, 3, h, m, s)
             checkEvaluation(
               Hour(Literal(new Timestamp(c.getTimeInMillis)), timeZoneId),