Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/boxingOptimization/fold.kt

@@ -0,0 +1,14 @@
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun box() : String {
+    val x = LongArray(5)
+    for (i in 0..4) {
+        x[i] = (i + 1).toLong()
+    }
+
+    assertEquals(15L, x.fold(0L) { x, y -> x + y })
+
+    return "OK"
+}