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).

test/test_helper.rb

@@ -30,11 +30,13 @@ class ActiveSupport::TestCase
   mock_post_version_service!
   mock_pool_version_service!
 
-  parallelize
-  parallelize_setup do |worker|
-    Rails.application.load_seed
+  unless Danbooru.config.debug_mode
+    parallelize
+    parallelize_setup do |worker|
+      Rails.application.load_seed
 
-    SimpleCov.command_name "#{SimpleCov.command_name}-#{worker}"
+      SimpleCov.command_name "#{SimpleCov.command_name}-#{worker}"
+    end
   end
 
   parallelize_teardown do |worker|