lib/fs: make watcher tests even more darwin slowness resistant

[imsodin]

Apparently I can't push to master, so lets go via PR.

$ git push upstream master 
Counting objects: 5, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (5/5), 468 bytes | 468.00 KiB/s, done.
Total 5 (delta 4), reused 0 (delta 0)
remote: Resolving deltas: 100% (4/4), completed with 4 local objects.
remote: error: GH006: Protected branch update failed for refs/heads/master.
remote: error: You're not authorized to push to this branch. Visit https://help.github.com/articles/about-protected-branches/ for more information.
To github.com:syncthing/syncthing.git
 ! [remote rejected]   master -> master (protected branch hook declined)
error: failed to push some refs to 'git@github.com:syncthing/syncthing.git'

[imsodin]

@st-review merge

[st-review]

@imsodin: Build status is pending. I'll wait until it goes green and then merge!

[st-review]

👌 Merged as e700037. Thanks, @imsodin!