[tests] Reduce number of threads in ffi/many_listener_callbacks_test

Issue: https://github.com/dart-lang/sdk/issues/61733
Change-Id: I3d6ef801f7c7f24d56b129ac4f169a7b2a2c5448
Cq-Include-Trybots: luci.dart.try:vm-ffi-mac-debug-simarm64_arm64-try,vm-ffi-mac-release-simarm64_arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/455464
Commit-Queue: Liam Appelbe <liama@google.com>
Reviewed-by: Liam Appelbe <liama@google.com>
Auto-Submit: Alexander Markov <alexmarkov@google.com>

tests/ffi/many_listener_callbacks_test.dart

@@ -54,7 +54,7 @@ void main() async {
 
   await firstIsRunning.future;
 
-  for (var i = 0; i < 30; ++i) {
+  for (var i = 0; i < 10; ++i) {
     spawn((int n) {});
   }