Minor, move builtins serializer test data

compiler/testData/serialization/builtinsSerializer/simple.kt

@@ -0,0 +1,11 @@
+package test
+
+class Class {
+    fun member() = null
+}
+
+fun function(int: Int, string: String = "default"): Class
+
+fun <T> T.extension(): T?
+
+val property: Unit