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/propertyInConstructor.kt

@@ -0,0 +1,11 @@
+trait A<T> {
+    var x: T
+}
+
+class B(override var x: String) : A<String>
+
+fun box(): String {
+    val a: A<String> = B("Fail")
+    a.x = "OK"
+    return a.x
+}