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/propertySetter.kt

@@ -0,0 +1,13 @@
+trait A<T> {
+    var v: T
+}
+
+class B : A<String> {
+    override var v: String = "Fail"
+}
+
+fun box(): String {
+    val a: A<String> = B()
+    a.v = "OK"
+    return a.v
+}