nixos/tests/sway: add check that sway quits
nixos/tests/sway: add check that sway quits

We used to check that sway quits _succesfully_.
However, since 73d7f08b4d89b1af213db5db34e6f39518d88634
disabled this, we'll add another check to see it quits _at all_.

cc @primeos

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