Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/functions/defaultargs.kt

@@ -0,0 +1,12 @@
+open abstract class B() : java.lang.Object() {
+    fun foo(arg: Int = 239 + 1) : Int = arg
+}
+
+class C() : B() {
+}
+
+fun box() : String {
+    if(C().foo(10) != 10) return "fail"
+    if(C().foo() != 240) return "fail"
+    return "OK"
+}