The line filter parsing added to ActiveSupport::TestCase is only half the story
to enable line filtering. The other half, of adding the patterns to the options,
is done in the Minitest plugin that Railties has.

Thus it makes more sense to have the filter in Railties with the other half and
all the line filtering tests.

Move the filter and extend Active Support in an initializer, so that when users
or `rails/all.rb` require `rails/test_unit/railtie` we can still filter by line.