[SPARK-22595][SQL] fix flaky test: CastSuite.SPARK-22500: cast for struct...

[SPARK-22595][SQL] fix flaky test: CastSuite.SPARK-22500: cast for struct should not generate codes beyond 64KB

## What changes were proposed in this pull request?

This PR reduces the number of fields in the test case of `CastSuite` to fix an issue that is pointed at [here](https://github.com/apache/spark/pull/19800#issuecomment-346634950).

```
java.lang.OutOfMemoryError: GC overhead limit exceeded
java.lang.OutOfMemoryError: GC overhead limit exceeded
	at org.codehaus.janino.UnitCompiler.findClass(UnitCompiler.java:10971)
	at org.codehaus.janino.UnitCompiler.findTypeByName(UnitCompiler.java:7607)
	at org.codehaus.janino.UnitCompiler.getReferenceType(UnitCompiler.java:5758)
	at org.codehaus.janino.UnitCompiler.getType2(UnitCompiler.java:5732)
	at org.codehaus.janino.UnitCompiler.access$13200(UnitCompiler.java:206)
	at org.codehaus.janino.UnitCompiler$18.visitReferenceType(UnitCompiler.java:5668)
	at org.codehaus.janino.UnitCompiler$18.visitReferenceType(UnitCompiler.java:5660)
	at org.codehaus.janino.Java$ReferenceType.accept(Java.java:3356)
	at org.codehaus.janino.UnitCompiler.getType(UnitCompiler.java:5660)
	at org.codehaus.janino.UnitCompiler.buildLocalVariableMap(UnitCompiler.java:2892)
	at org.codehaus.janino.UnitCompiler.compile(UnitCompiler.java:2764)
	at org.codehaus.janino.UnitCompiler.compileDeclaredMethods(UnitCompiler.java:1262)
	at org.codehaus.janino.UnitCompiler.compileDeclaredMethods(UnitCompiler.java:1234)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:538)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:890)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:894)
	at org.codehaus.janino.UnitCompiler.access$600(UnitCompiler.java:206)
	at org.codehaus.janino.UnitCompiler$2.visitMemberClassDeclaration(UnitCompiler.java:377)
	at org.codehaus.janino.UnitCompiler$2.visitMemberClassDeclaration(UnitCompiler.java:369)
	at org.codehaus.janino.Java$MemberClassDeclaration.accept(Java.java:1128)
	at org.codehaus.janino.UnitCompiler.compile(UnitCompiler.java:369)
	at org.codehaus.janino.UnitCompiler.compileDeclaredMemberTypes(UnitCompiler.java:1209)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:564)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:890)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:894)
	at org.codehaus.janino.UnitCompiler.access$600(UnitCompiler.java:206)
	at org.codehaus.janino.UnitCompiler$2.visitMemberClassDeclaration(UnitCompiler.java:377)
	at org.codehaus.janino.UnitCompiler$2.visitMemberClassDeclaration(UnitCompiler.java:369)
	at org.codehaus.janino.Java$MemberClassDeclaration.accept(Java.java:1128)
	at org.codehaus.janino.UnitCompiler.compile(UnitCompiler.java:369)
	at org.codehaus.janino.UnitCompiler.compileDeclaredMemberTypes(UnitCompiler.java:1209)
	at org.codehaus.janino.UnitCompiler.compile2(UnitCompiler.java:564)
...
```

## How was this patch tested?

Used existing test case

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #19806 from kiszk/SPARK-22595.

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

@@ -829,7 +829,7 @@ class CastSuite extends SparkFunSuite with ExpressionEvalHelper {
   }
 
   test("SPARK-22500: cast for struct should not generate codes beyond 64KB") {
-    val N = 250
+    val N = 25
 
     val fromInner = new StructType(
       (1 to N).map(i => StructField(s"s$i", DoubleType)).toArray)