Merge boxWithJava testData into box, delete BoxWithJava test

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

@@ -0,0 +1,19 @@
+// WITH_REFLECT
+// FILE: J.java
+
+public class J {
+    private String result = "Fail";
+}
+
+// FILE: K.kt
+
+import kotlin.reflect.*
+import kotlin.reflect.jvm.*
+
+fun box(): String {
+    val a = J()
+    val p = J::class.members.single { it.name == "result" } as KMutableProperty1<J, String>
+    p.isAccessible = true
+    p.set(a, "OK")
+    return p.get(a)
+}