[mjsunit] Check array length in array-concat

The speedup in https://crrev.com/c/2504853 left out checking the array
lengths, this adds that check.

Bug: v8:7783
Change-Id: I8de01fa2dff3e051246ee3a268cdce2128ad16ea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2505252Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70869}

test/mjsunit/array-concat.js

@@ -238,6 +238,8 @@ expected[largeLength + 2] = "X";
 var numElementsToCheck = 10;
 
 // Checking entire massive array is too slow, so check:
+// - the length,
+assertEquals(expected.length, r3.length);
 var slicesToCheck = [
   // - the first few elements,
   {start: 0, end: initialLen},