test: tune test-uv-threadpool-schedule

test-uv-threadpool-schedule has been failing consistently in
node-daily-master CI. It also fails consistently on my personal laptop.
These changes make it pass consistently with current master but fail
consistently with Node.js 10.11 (which was the last release in the 10.x
line before the fix that the test is for). It succeeds/fails as expected
whether or not I am connected to the network.

Fixes: https://github.com/nodejs/node/issues/25305

PR-URL: https://github.com/nodejs/node/pull/25358


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>