Merge boxWithStdlib testData into box, delete BoxWithStdlib test

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

@@ -0,0 +1,13 @@
+// WITH_RUNTIME
+// See KT-10107: 'Variable must be initialized' for delegate with private set
+
+class My {
+    var delegate: String by kotlin.properties.Delegates.notNull()
+        private set
+
+    init {
+        delegate = "OK"
+    }
+}
+
+fun box() = My().delegate