Use kts file extension instead of kt in tests...

since all "kt" files are now considered - definitely non-scripts

compiler/testData/codegen/scriptCustom/fibwp.lang.kts

@@ -0,0 +1,11 @@
+package test
+
+// this script expected parameter num : Int
+
+fun fib(n: Int): Int {
+    val v = if(n < 2) 1 else fib(n-1) + fib(n-2)
+    return v
+}
+
+val result: Int = fib(num)
+12