Explicitly add testing email settings (#11685)

If you have a `.env` file in the repository that specifies mal settings,
these can overwrite the default mailing settings for the testing setup.
This commit explicitly sets the appropriate env variable correctly.

docker-compose.yml

@@ -172,6 +172,7 @@ services:
       CAPYBARA_SERVER_PORT: 3000
       CAPYBARA_DYNAMIC_BIND_IP: 1
       CAPYBARA_APP_HOSTNAME: backend-test
+      OPENPROJECT_EMAIL__DELIVERY__METHOD: test
       OPENPROJECT_CLI_PROXY: http://frontend-test:4200
       OPENPROJECT_TESTING_NO_HEADLESS: "true"
       OPENPROJECT_TESTING_AUTO_DEVTOOLS: "true"