Improve Action Text System Test coverage

Introduce the `ActionText::RichTextEditorTest` to exercise Action Text's
integration with Active Storage. First, assert that Action Text renders
an Active Storage blob preview of the image within the `<trix-editor>`
element prior to persisting the rich-text record. Next, assert that
Active Storage renders the uploaded file within the rendered rich-text
content.

In addition to expanding System Test coverage, this commit also alters
the existing `ActionText::SystemTestHelperTest` coverage to utilize
Capybara's built-in [:element][] selector as an alternative to CSS. The
`:element` selector supports arbitrary keyword arguments as HTML
attribute filters. For example, `input: true` as a filter translates to
asserting the presence of any `[input]` attribute. Similarly, it enables
assertions with Ruby strings instead of interpolating Ruby strings
**into** CSS selectors.

[:element]: https://rubydoc.info/github/teamcapybara/capybara/master/Capybara/Selector

Author: seanpdoyle

actiontext/test/system/rich_text_editor_test.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require "application_system_test_case"
+
+class ActionText::RichTextEditorTest < ApplicationSystemTestCase
+  test "attaches and uploads image file" do
+    image_file = file_fixture("racecar.jpg")
+
+    visit new_message_url
+    attach_file image_file do
+      click_button "Attach Files"
+    end
+    within :rich_text_area do
+      assert_selector :element, "img", src: %r{/rails/active_storage/blobs/redirect/.*/#{image_file.basename}\Z}
+    end
+    click_button "Create Message"
+
+    within class: "trix-content" do
+      assert_selector :element, "img", src: %r{/rails/active_storage/representations/redirect/.*/#{image_file.basename}\Z}
+    end
+  end
+end

actiontext/test/system/system_test_helper_test.rb

@@ -8,31 +8,31 @@ def setup
   end
 
   test "filling in a rich-text area by ID" do
-    assert_selector "trix-editor#message_content"
+    assert_selector :element, "trix-editor", id: "message_content"
     fill_in_rich_textarea "message_content", with: "Hello world!"
     assert_selector :field, "message[content]", with: /Hello world!/, type: "hidden"
   end
 
   test "filling in a rich-text area by placeholder" do
-    assert_selector "trix-editor[placeholder='Your message here']"
+    assert_selector :element, "trix-editor", placeholder: "Your message here"
     fill_in_rich_textarea "Your message here", with: "Hello world!"
     assert_selector :field, "message[content]", with: /Hello world!/, type: "hidden"
   end
 
   test "filling in a rich-text area by aria-label" do
-    assert_selector "trix-editor[aria-label='Message content aria-label']"
+    assert_selector :element, "trix-editor", "aria-label": "Message content aria-label"
     fill_in_rich_textarea "Message content aria-label", with: "Hello world!"
     assert_selector :field, "message[content]", with: /Hello world!/, type: "hidden"
   end
 
   test "filling in a rich-text area by label" do
-    assert_selector "label", text: "Message content label"
+    assert_selector :label, "Message content label", for: "message_content"
     fill_in_rich_textarea "Message content label", with: "Hello world!"
     assert_selector :field, "message[content]", with: /Hello world!/, type: "hidden"
   end
 
   test "filling in a rich-text area by input name" do
-    assert_selector "trix-editor[input]"
+    assert_selector :element, "trix-editor", input: true
     fill_in_rich_textarea "message[content]", with: "Hello world!"
     assert_selector :field, "message[content]", with: /Hello world!/, type: "hidden"
   end