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/defaultargs5.kt

@@ -0,0 +1,13 @@
+open abstract class B() : java.lang.Object() {
+    abstract fun foo2(arg: Int = 239) : Int
+}
+
+class C() : B() {
+    override fun foo2(arg: Int) : Int = arg
+}
+
+fun box() : String {
+    if(C().foo2() != 239) return "fail"
+    if(C().foo2(10) != 10) return "fail"
+    return "OK"
+}