This fixes a flaky test which has been occasionally timing out in CI.
In particular, this test times out if `watchFile` receives multiple
notifications from inotify before we cancel the test context. We block
processing the second notification, because we've stopped listening to
the `callbackDone` channel.
This patch changes the test so we only send on the first notification.
Testing this locally with `stress` confirms that the test is no longer
flaky.
Fixes#17172
Updates #14699
Signed-off-by: Alex Chan <alexc@tailscale.com>
