config: add debug_mode option.

Add a debug mode option. This is useful when debugging failed tests.

Debug mode disables parallel testing so you can set breakpoints in tests
with binding.pry (normally parallel testing makes it hard to set
breakpoints).

Debug mode also disables global exception handling for controllers. This
lets exceptions bubble up to the console during controller tests
(normally exceptions are swallowed by the controller, which prevents you
from seeing backtraces in failed controller tests).

config/danbooru_default_config.rb

@@ -52,6 +52,15 @@ module Danbooru
       "_danbooru2_session"
     end
 
+    # Debug mode does some things to make testing easier. It disables parallel
+    # testing and it replaces Danbooru's custom exception page with the default
+    # Rails exception page. This is only useful during development and testing.
+    #
+    # Usage: `DANBOORU_DEBUG_MODE=true bin/rails test
+    def debug_mode
+      false
+    end
+
     def source_code_url
       "https://github.com/danbooru/danbooru"
     end