[SPARK-23928][TESTS][FOLLOWUP] Set seed to avoid flakiness

## What changes were proposed in this pull request?

The tests for shuffle can be flaky (eg. https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/94355/testReport/). This happens because we have not set the seed for `Random`.

## How was this patch tested?

running 10000 times the UT (validated that with a different seed eg. 12345 the test fails).

Closes #22023 from mgaido91/SPARK-23928_followup.

Authored-by: Marco Gaido <marcogaido91@gmail.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

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

@@ -1489,7 +1489,7 @@ class CollectionExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper
     checkEvaluation(Shuffle(as7, Some(0)), null)
     checkEvaluation(Shuffle(aa, Some(0)), Seq(Seq("e"), Seq("a", "b"), Seq("c", "d")))
 
-    val r = new Random()
+    val r = new Random(1234)
     val seed1 = Some(r.nextLong())
     assert(evaluateWithoutCodegen(Shuffle(ai0, seed1)) ===
       evaluateWithoutCodegen(Shuffle(ai0, seed1)))