tests: add a smoke test

nix-build tests -A smoke runs 19 scenarios in a single VM test: each
source type, each kind of target, flakes drawing inputs both ways, the
sentinel, prune, the checks of source names and target paths, and
writeDeploy.  That's quick enough to run after every change, while the
complete tests stay what a change has to pass in the end.  nix-build
tests doesn't build it, because the complete tests run the same
scenarios.

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