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/delegationProperty.kt

@@ -0,0 +1,15 @@
+trait A<T> {
+    var result: T
+}
+
+class B(a: A<String>): A<String> by a
+
+fun box(): String {
+    val o = object : A<String> {
+        override var result = "Fail"
+    }
+    val b: A<String> = B(o)
+    b.result = "OK"
+    if (b.result != "OK") return "Fail"
+    return (b : A<String>).result
+}