Add a system spec for the /share endpoint (#28672)

spec/system/share_entrypoint_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'ShareEntrypoint' do
+  include ProfileStories
+
+  subject { page }
+
+  let(:email)               { 'test@example.com' }
+  let(:password)            { 'password' }
+  let(:confirmed_at)        { Time.zone.now }
+  let(:finished_onboarding) { true }
+
+  before do
+    as_a_logged_in_user
+    visit share_path
+  end
+
+  it 'can be used to post a new status' do
+    expect(subject).to have_css('div#mastodon-compose')
+    expect(subject).to have_css('.compose-form__publish-button-wrapper > button')
+
+    status_text = 'This is a new status!'
+
+    within('.compose-form') do
+      fill_in "What's on your mind?", with: status_text
+      click_on 'Publish!'
+    end
+
+    expect(subject).to have_css('.notification-bar-message', text: 'Post published.')
+  end
+end