[mjsunit] Make an Array#join regression test 8x faster

regress-336820 is testing that joining a very sparse
array to create a too-big string results in a RangeError,
rather than a crash. Reducing the largest index by
two orders of magnitude speeds this up (on x64 debug)
by 8x (from 8 seconds down to 1). Given that this test
takes nearly 9 minutes on arm64 sim debug, I hope to
see big ones there too.

Bug: v8:7783, chromium:336820
Change-Id: I74c22cf451a892eb039efc7f1259152921bf8530
Reviewed-on: https://chromium-review.googlesource.com/c/1323915Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57335}

test/mjsunit/regress/regress-336820.js

@@ -33,6 +33,6 @@ assertThrows((function() {
     x = new Array();
     x[0] = s;
     x[1000] = s;
-    x[1000000] = s;
+    x[10000] = s;
     s = x.join("::");
   }}), RangeError);