[SPARK-22499][FOLLOWUP][SQL] Reduce input string expressions for Least and Greatest to reduce time in its test

## What changes were proposed in this pull request?

It's minor and trivial but looks 2000 input is good enough to reproduce and test in SPARK-22499.

## How was this patch tested?

Manually brought the change and tested.

Locally tested:

Before: 3m 21s 288ms
After: 1m 29s 134ms

Given the latest successful build took:

```
ArithmeticExpressionSuite:
- SPARK-22499: Least and greatest should not generate codes beyond 64KB (7 minutes, 49 seconds)
```

I expect it's going to save 4ish mins.

Author: hyukjinkwon <[email protected]>

Closes apache#21855 from HyukjinKwon/minor-fix-suite.

Author: HyukjinKwon

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

@@ -349,7 +349,7 @@ class ArithmeticExpressionSuite extends SparkFunSuite with ExpressionEvalHelper
   }
 
   test("SPARK-22499: Least and greatest should not generate codes beyond 64KB") {
-    val N = 3000
+    val N = 2000
     val strings = (1 to N).map(x => "s" * x)
     val inputsExpr = strings.map(Literal.create(_, StringType))