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/inheritedMethod.kt

@@ -0,0 +1,13 @@
+open class Foo {
+  fun xyzzy(): String = "xyzzy"
+}
+
+class Bar(): Foo() {
+  fun test(): String = xyzzy()
+}
+
+fun box() : String {
+  val bar = Bar()
+  val f = bar.test()
+  return if (f == "xyzzy") "OK" else "fail"
+}