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/propertyAccessorsWithoutBody.kt

@@ -0,0 +1,15 @@
+open class A<T> {
+    open var x: T = "Fail" as T
+    get
+}
+
+class B : A<String>() {
+    override var x: String = "Fail"
+    set
+}
+
+fun box(): String {
+    val a: A<String> = B()
+    a.x = "OK"
+    return a.x
+}