Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/reified/instanceof.kt

@@ -0,0 +1,13 @@
+// WITH_RUNTIME
+
+inline fun<reified T> isinstance(x: Any?): Boolean {
+    return x is T
+}
+
+fun box(): String {
+    assert(isinstance<String>("abc"))
+    assert(isinstance<Int>(1))
+    assert(!isinstance<Int>("abc"))
+
+    return "OK"
+}