parameters.yml:
time_limit: 8
my_ui.yml:
my_ui:
time_limit: %time_limit%
config.yml:
The best way I found is setting up your own functional test environment for testing. This is completely seperated from your prod / dev environment.
You can study nice examplse of this approach in Johann Schmidts bundles. I copied and adapted the one from the JMSPaymentCoreBundle for my projects.
An other approach is to include (and override prod and dev settings) int he config_test.yml file. This file should be loaded by the test client only.