lib/tests: Add check-eval.nix to run simple tests.

This can be used by evaluation-only tools to validate tests are still
working.

Changed files
+7
lib
+7
lib/tests/check-eval.nix
···
+
# Throws an error if any of our lib tests fail.
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+
let tests = [ "misc" "systems" ];
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all = builtins.concatLists (map (f: import (./. + "/${f}.nix")) tests);
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in if all == []
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then null
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else throw (builtins.toJSON all)