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/kt2566.kt

@@ -0,0 +1,15 @@
+open class A {
+    open fun foo() = "OK"
+}
+
+open class B : A() {
+    override fun foo() = super.foo()
+}
+
+trait I
+
+class C : I, B() {
+    override fun foo() = super<B>.foo()
+}
+
+fun box() = C().foo()