Minor, move builtins serializer test data

compiler/testData/serialization/builtinsSerializer/simple.txt

@@ -0,0 +1,10 @@
+package test
+
+internal val property: kotlin.Unit
+internal fun function(/*0*/ int: kotlin.Int, /*1*/ string: kotlin.String = ...): test.Class
+internal fun </*0*/ T> T.extension(): T?
+
+internal final class Class {
+    public constructor Class()
+    internal final fun member(): kotlin.Nothing?
+}