Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/traits/kt2963.kt

@@ -0,0 +1,15 @@
+open class Base
+
+trait Derived : Base {
+    fun foo(): String {
+        return object {
+            fun bar() = baz(this@Derived)
+        }.bar()
+    }
+}
+
+class DerivedImpl : Derived, Base()
+
+fun baz(b: Base) = "OK"
+
+fun box() = DerivedImpl().foo()