[VM/Service] Fix get_perfetto_cpu_samples_rpc_test

This test recently flipped back to flaky on vm-linux-debug-ia32. I
believe the problem is that calculating `fib(44)` takes a long time on
less powerful machines, which can cause enough samples to get collected
to fill up the sample buffer and cause data to get overwritten.

Issue: #54401
Change-Id: I63b0bdca5b04b066db098c12cfc22845a9b3c9dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410781
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Derek Xu <[email protected]>

Author: derekxu16

pkg/vm_service/test/get_perfetto_cpu_samples_rpc_test.dart

@@ -19,7 +19,7 @@ int fib(n) {
 
 void testeeDo() {
   print('Testee doing something.');
-  fib(44);
+  fib(21);
   print('Testee did something.');
 }