[cpu-profiler] Speed up slow test for arm simulator

Do less work in MultipleProfilers. Reduces runtime from ~8 mins to ~40
seconds.

Bug: v8:8474
Change-Id: I72b3266941ce40c8d064deaf00fb06f8d9fa8a70
Reviewed-on: https://chromium-review.googlesource.com/c/1341956Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57613}

test/cctest/test-cpu-profiler.cc

@@ -2561,9 +2561,9 @@ void ProfileSomeCode(v8::Isolate* isolate) {
   const char* source = R"(
       function foo() {
         var x = 0;
-        for (var i = 0; i < 1e4; i++) {
+        for (var i = 0; i < 1e3; i++) {
           for (var j = 0; j < 1e3; j++) {
-            x += i * j;
+            x = i * j;
           }
         }
         return x;