Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/delegatedProperty/protectedVarWithPrivateSet.kt

@@ -0,0 +1,16 @@
+// WITH_RUNTIME
+
+import kotlin.properties.Delegates
+
+open class A<T : Any> {
+    protected var value: T by Delegates.notNull()
+        private set
+}
+
+class B : A<Int>()
+
+fun box(): String {
+    B()
+
+    return "OK"
+}