Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/bridges/kt2498.kt

@@ -0,0 +1,19 @@
+import java.util.ArrayList
+
+open class BaseStringList: ArrayList<String>() {
+}
+
+class StringList: BaseStringList() {
+    public override fun get(index: Int): String {
+        return "StringList.get()"
+    }
+}
+
+fun box(): String {
+    val myStringList = StringList()
+    myStringList.add("first element")
+    if (myStringList.get(0) != "StringList.get()") return "Fail #1"
+    if ((myStringList: BaseStringList).get(0) != "StringList.get()") return "Fail #2"
+    if ((myStringList: ArrayList<String>).get(0) != "StringList.get()") return "Fail #3"
+    return "OK"
+}