Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/reflection/properties/getExtensionPropertiesMutableVsReadonly.kt

@@ -0,0 +1,28 @@
+// WITH_REFLECT
+
+import kotlin.reflect.*
+
+var storage = "before"
+
+class A {
+    val String.readonly: String
+        get() = this
+
+    var String.mutable: String
+        get() = storage
+        set(value) { storage = value }
+}
+
+fun box(): String {
+    val props = A::class.java.kotlin.memberExtensionProperties
+    val readonly = props.single { it.name == "readonly" }
+    assert(readonly !is KMutableProperty2<A, *, *>) { "Fail 1: $readonly" }
+    val mutable = props.single { it.name == "mutable" }
+    assert(mutable is KMutableProperty2<A, *, *>) { "Fail 2: $mutable" }
+
+    val a = A()
+    mutable as KMutableProperty2<A, String, String>
+    assert(mutable.get(a, "") == "before") { "Fail 3: ${mutable.get(a, "")}" }
+    mutable.set(a, "", "OK")
+    return mutable.get(a, "")
+}