Added test for settings.environment in views

Adding a test to prove issue https://github.com/PerlDancer/Dancer2/issues/650 . If the environment config setting is not set explicitly in the config or inside the app (via the 'set' keyword'), the 'settings.environment' variable inside the view is empty (no default value is set apparently). However, if the environment variable is set explicitly, the 'settings.environment' value is accessible in the view.