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/inheritedInnerClass.kt

@@ -0,0 +1,14 @@
+class Outer() {
+  open inner class InnerBase() {
+  }
+
+  inner class InnerDerived(): InnerBase() {
+  }
+
+  public val foo: InnerBase? = InnerDerived()
+}
+
+fun box() : String {
+  val o = Outer()
+  return if (o.foo === null) "fail" else "OK"
+}