Move blackBoxFile() testData to box/ directory

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

compiler/testData/codegen/box/bridges/delegation.kt

@@ -0,0 +1,11 @@
+trait A<T> {
+    fun foo(): T
+}
+
+class B : A<String> {
+    override fun foo() = "OK"
+}
+
+class C(a: A<String>) : A<String> by a
+
+fun box() = (C(B()) : A<String>).foo()