Wait for js component to mount before editing (#17239)

This test had been failing with "title can't be blank" a few times
recently. Looking for a difference between the v2 form template (in
app/views/articles) and the ArticleForm component (in
app/javascript/article-form/) showed there is an id on the js form but
not the erb/html form we respond with first.

The test had been sometimes typing the title into the text area before
the component mounts, and trying to view the article gives a
validation error rather than the expected response.

spec/system/user_uses_the_editor_spec.rb

@@ -98,11 +98,12 @@ RSpec.describe "Using the editor", type: :system do
   describe "using v2 editor", js: true do
     it "fill out form with rich content and click publish" do
       visit "/new"
-      fill_in "article-form-title", with: "This is a <span> test"
-      fill_in "tag-input", with: "What, Yo"
-      fill_in "article_body_markdown", with: "Hello"
+      within "form#article-form" do
+        fill_in "article-form-title", with: "This is a <span> test"
+        fill_in "tag-input", with: "What, Yo"
+        fill_in "article_body_markdown", with: "Hello"
+      end
       find("button", text: /\APublish\z/).click
-
       expect(page).to have_xpath("//div[@class='crayons-article__header__meta']//h1")
       expect(page).to have_text("Hello")
       expect(page).to have_link("what", href: "/t/what")