[SPARK-25605][TESTS] Run cast string to timestamp tests for a subset of timezones

## What changes were proposed in this pull request?

The test `cast string to timestamp` used to run for all time zones. So it run for more than 600 times. Running the tests for a significant subset of time zones is probably good enough and doing this in a randomized manner enforces anyway that we are going to test all time zones in different runs.

## How was this patch tested?

the test time reduces to 11 seconds from more than 2 minutes

Closes apache#22631 from mgaido91/SPARK-25605.

Authored-by: Marco Gaido <[email protected]>
Signed-off-by: gatorsmile <[email protected]>

Author: mgaido91

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

@@ -20,6 +20,8 @@ package org.apache.spark.sql.catalyst.expressions
 import java.sql.{Date, Timestamp}
 import java.util.{Calendar, Locale, TimeZone}
 
+import scala.util.Random
+
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.Row
 import org.apache.spark.sql.catalyst.InternalRow
@@ -110,7 +112,7 @@ class CastSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 
   test("cast string to timestamp") {
-    for (tz <- ALL_TIMEZONES) {
+    for (tz <- Random.shuffle(ALL_TIMEZONES).take(50)) {
       def checkCastStringToTimestamp(str: String, expected: Timestamp): Unit = {
         checkEvaluation(cast(Literal(str), TimestampType, Option(tz.getID)), expected)
       }