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/defaultargs3.kt

@@ -0,0 +1,15 @@
+
+class C() {
+    fun Any.toMyPrefixedString(prefix: String = "", suffix: String="") : String = prefix + " " + suffix
+
+    fun testReceiver() : String {
+        val res : String = "mama".toMyPrefixedString("111", "222")
+        return res
+    }
+
+}
+
+fun box() : String {
+    if(C().testReceiver() != "111 222") return "fail"
+    return "OK"
+}