tsnet: add tests to TestListenService for user-supplied TUN devices

This resolves a gap in test coverage, ensuring Server.ListenService
functions as expected in combination with user-supplied TUN devices

Fixes tailscale/corp#36603

Co-authored-by: Harry Harpham <harry@tailscale.com>
Signed-off-by: Harry Harpham <harry@tailscale.com>
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