DEV: Speed up chat plugin system tests (#21399)

See e323628d8a363ea29d1f341b95ffcdf8f3c09fca for more details.

This commit speeds up the tests by roughly 10 seconds locally where the
default wait time is 2 seconds. On CI, this speeds up the tests by 20
seconds where the default wait time is 4 seconds.

plugins/chat/spec/system/create_channel_spec.rb

@@ -10,11 +10,12 @@ RSpec.describe "Create channel", type: :system, js: true do
 
   context "when user cannot create channel" do
     fab!(:current_user) { Fabricate(:user) }
+
     before { sign_in(current_user) }
 
     it "does not show the create channel button" do
       chat_page.visit_browse
-      expect(chat_page).not_to have_new_channel_button
+      expect(chat_page).to have_no_new_channel_button
     end
   end