[test] Fix NNBD version of spawn_shutdown_test.

Change-Id: If871bd816901a9f5b174ecc6c217269935631af7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/281305
Reviewed-by: Brian Quinlan <bquinlan@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>