test: rename scenarios/lib.py -> base.py

This file isn't a scenario, it's also not a lib because it contains
the main share of actual tests.

test/test.nix

@@ -57,6 +57,5 @@ import ./make-test.nix rec {
     '';
   };
   testScript =
-    builtins.readFile ./scenarios/lib.py + "\n\n" +
+    builtins.readFile ./base.py + "\n\n" + builtins.readFile "${./.}/scenarios/${scenario}.py";
-    builtins.readFile "${./.}/scenarios/${scenario}.py";
 }