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/kt2607.kt

@@ -0,0 +1,9 @@
+fun box() : String {
+    val o = object {
+
+        inner class C {
+            fun foo() = "OK"
+        }
+    }
+    return o.C().foo()
+}