[VM/Service] Fix get_perfetto_cpu_samples_rpc_test

My change
f3536c5
has made the test start failing on more configs, because now the `fib`
computation isn't lasting long enough for any samples to get collected.
This change makes the `fib` computation take longer.

Issue: #54401
Change-Id: I0f038df21bb116f97a5093e7d2e58ccda7447977
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411161
Commit-Queue: Derek Xu <[email protected]>
Reviewed-by: Liam Appelbe <[email protected]>

Author: derekxu16

pkg/vm_service/test/get_perfetto_cpu_samples_rpc_test.dart

@@ -11,15 +11,12 @@ import 'package:vm_service_protos/vm_service_protos.dart';
 import 'common/service_test_common.dart';
 import 'common/test_helper.dart';
 
-int fib(n) {
-  if (n < 0) return 0;
-  if (n == 0) return 1;
-  return fib(n - 1) + fib(n - 2);
-}
-
 void testeeDo() {
   print('Testee doing something.');
-  fib(21);
+  final stopwatch = Stopwatch();
+  stopwatch.start();
+  while (stopwatch.elapsedMilliseconds < 5000) {}
+  stopwatch.stop();
   print('Testee did something.');
 }