tests/test-derivation: don't invoke test modules

`def.module or lib.removeAttrs def [ "tests" ]` is actually equivalent to
`(def.module or lib.removeAttrs) def [ "tests" ]`.

This meant whenever `def` had a `module` attribute, it was invoked as
`def.module def [ "tests" ]`!

tests/default.nix

@@ -37,7 +37,7 @@ let
       inherit name;
       # The module can either be the actual definition,
       # or a child attr named `module`.
-      module = def.module or lib.removeAttrs def [ "tests" ];
+      module = def.module or (lib.removeAttrs def [ "tests" ]);
       dontRun = def.tests.dontRun or false;
       pkgs = pkgsUnfree;
     };