Use `on_load` to trigger commandline processing code

We need to use on_load so that plugins will get the same functionality

activesupport/lib/active_support/test_case.rb

@@ -65,5 +65,7 @@ module ActiveSupport
     alias :assert_not_predicate :refute_predicate
     alias :assert_not_respond_to :refute_respond_to
     alias :assert_not_same :refute_same
+
+    ActiveSupport.run_load_hooks(:active_support_test_case, self)
   end
 end

railties/lib/rails/commands/test/test_command.rb

@@ -1,6 +1,5 @@
 require "rails/command"
 require "rails/test_unit/minitest_plugin"
-require "rails/test_unit/line_filtering"
 
 module Rails
   module Command
@@ -12,10 +11,6 @@ module Rails
       def perform(*)
         $LOAD_PATH << Rails::Command.root.join("test")
 
-        # Add test line filtering support for running test by line number
-        # via the command line.
-        ActiveSupport::TestCase.extend Rails::LineFiltering
-
         Minitest.run_via[:rails] = true
 
         require "active_support/testing/autorun"

railties/lib/rails/test_unit/railtie.rb

@@ -1,3 +1,5 @@
+require "rails/test_unit/line_filtering"
+
 if defined?(Rake.application) && Rake.application.top_level_tasks.grep(/^(default$|test(:|$))/).any?
   ENV["RAILS_ENV"] ||= "test"
 end
@@ -11,6 +13,12 @@ module Rails
       c.integration_tool :test_unit
     end
 
+    initializer "test_unit.line_filtering" do
+      ActiveSupport.on_load(:active_support_test_case) {
+        ActiveSupport::TestCase.extend Rails::LineFiltering
+      }
+    end
+
     rake_tasks do
       load "rails/test_unit/testing.rake"
     end