test: add option `extraTestScript`

This allows extending tests.py.

test/lib/make-test.nix

@@ -42,6 +42,7 @@ name: testConfig:
       builtins.concatStringsSep "\n\n" [
         initData
         (builtins.readFile ./../tests.py)
+        cfg.test.extraTestScript
         # Don't run tests in interactive mode.
         # is_interactive is set in ../run-tests.sh
         ''

test/lib/test-lib.nix

@@ -19,7 +19,11 @@ with lib;
           dictionary variable 'test_data'. The data is exported via JSON.
         '';
       };
-
+      extraTestScript = mkOption {
+        type = types.lines;
+        default = "";
+        description = "Extra lines added to the Python test script.";
+      };
       container = {
         # Forwarded to extra-container. For descriptions, see
         # https://github.com/erikarvstedt/extra-container/blob/master/eval-config.nix