Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/arrays/kt7338.kt

@@ -0,0 +1,9 @@
+// WITH_RUNTIME
+
+fun foo(x : Any): String {
+    return if(x is Array<*> && x.isArrayOf<String>()) (x as Array<String>)[0] else "fail"
+}
+
+fun box(): String {
+    return foo(arrayOf("OK"))
+}