Speed up a new regression test

The test takes several minutes on slower bots, so speed it up a bit
without removing the ability to hit the data race.

R=ulan@chromium.org

Bug: chromium:1205290, v8:11741
Change-Id: I57e411bfa2ff2a22bef1a916b74f7684b2f0be17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2876855Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74408}

test/mjsunit/regress/regress-1205290.js

@@ -9,14 +9,15 @@ function waitForWorker() {
 function onmessage([sab, lock]) {
   const i32a = new Int32Array(sab);
   Atomics.store(lock, 0, 1);
-  for (let j = 1; j < 5000; ++j) {
+  for (let j = 1; j < 1000; ++j) {
     for (let i = 0; i < i32a.length; ++i) {
       i32a[i] = j;
     }
   }
 }
 const worker = new Worker(`onmessage = ${onmessage}`, {type: 'string'});
-const arr = new Int32Array(new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT * 2000));
+const arr =
+    new Int32Array(new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT * 100));
 worker.postMessage([arr.buffer, sync_arr]);
 
 waitForWorker();