Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/classes/kt2395.kt

@@ -0,0 +1,11 @@
+import java.util.AbstractList
+
+class MyList(): AbstractList<String>() {
+     public fun getModificationCount(): Int = modCount
+     public override fun get(index: Int): String = ""
+     public override fun size(): Int = 0
+}
+
+fun box(): String {
+    return if (MyList().getModificationCount() == 0) "OK" else "fail"
+}