Use unique controller per ActionView::TestCase

Previously the same class, ActionView::TestCase::TestController, was
used to build a controller for every ActionView::TestCase class.

This caused issues when helpers/helper methods were set directly on the
controller (which from one test we seem to want to support).

This commit solves this by creating a new controller class for every
test case, which gives the controller a unique set of helpers to work
with.
Co-authored-by: NJohn Crepezzi <seejohnrun@github.com>

actionpack/lib/action_controller/metal.rb

@@ -126,7 +126,7 @@ class Metal < AbstractController::Base
     # ==== Returns
     # * <tt>string</tt>
     def self.controller_name
-      @controller_name ||= name.demodulize.delete_suffix("Controller").underscore
+      @controller_name ||= (name.demodulize.delete_suffix("Controller").underscore unless anonymous?)
     end
 
     def self.make_response!(request)

actionview/lib/action_view/test_case.rb

@@ -16,11 +16,12 @@ class TestController < ActionController::Base
       attr_accessor :request, :response, :params
 
       class << self
-        attr_writer :controller_path
+        # Overrides AbstractController::Base#controller_path
+        attr_accessor :controller_path
       end
 
       def controller_path=(path)
-        self.class.controller_path = (path)
+        self.class.controller_path = path
       end
 
       def initialize
@@ -101,7 +102,7 @@ def include_helper_modules!
       end
 
       def setup_with_controller
-        @controller = ActionView::TestCase::TestController.new
+        @controller = Class.new(ActionView::TestCase::TestController).new
         @request = @controller.request
         @view_flow = ActionView::OutputFlow.new
         # empty string ensures buffer has UTF-8 encoding as

actionview/test/template/test_case_test.rb

@@ -156,6 +156,13 @@ def render_from_helper
 
       assert_equal "controller_helper_method", some_method
     end
+
+    class AnotherTestClass < ActionView::TestCase
+      test "doesn't use controller helpers from other tests" do
+        assert_not_respond_to view, :render_from_helper
+        assert_not_includes @controller._helpers.instance_methods, :render_from_helper
+      end
+    end
   end
 
   class ViewAssignsTest < ActionView::TestCase