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

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

lib/tests/check-eval.nix

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