device: fix goroutine leak test

device: fix goroutine leak test

The leak test had rare flakes.
If a system goroutine started at just the wrong moment, you'd get a false positive.
Instead of looping until the goroutines look good and then checking,
exit completely as soon as the number of goroutines looks good.
Also, check more frequently, in an attempt to complete faster.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>