[vm/benchmarks] Reduce number of isolate spawn runs from 5 to 3.

This is to reduce load on Android benchmarking hardware.

Change-Id: Ib51b3fbbe2bf66cb0ba17375062a747f37284299
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124600
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>

benchmarks/IsolateSpawnMemory/dart/IsolateSpawnMemory.dart

@@ -33,7 +33,7 @@ class SpawnMemory {
   SpawnMemory(this.name, this.wsUri, this.groupRefId);
 
   Future<void> report() async {
-    const numberOfRuns = 5;
+    const numberOfRuns = 3;
 
     int sumDeltaRssOnStart = 0;
     int sumDeltaRssOnEnd = 0;