tests.pkgs-lib.formats: Detect when impossible input is fed

pkgs/pkgs-lib/tests/formats.nix

@@ -9,7 +9,11 @@ let
     let
       formatSet = format args;
       config = formatSet.type.merge [] (imap1 (n: def: {
-        value = def;
+        # We check the input values, so that
+        #  - we don't write nonsensical tests that will impede progress
+        #  - the test author has a slightly more realistic view of the
+        #    final format during development.
+        value = lib.throwIfNot (formatSet.type.check def) (builtins.trace def "definition does not pass the type's check function") def;
         file = "def${toString n}";
       }) [ def ]);
     in formatSet.generate "test-format-file" config;