DEV: Improve performance of system test sign_in helper (#19579)

Previously, calling `sign_in` would cause the browser to be redirected to `/`, and would cause the Ember app to boot. We would then call `visit()`, causing the app to boot for a second time.

This commit adds a `redirect=false` option to the `/session/username/become` route. This avoids the unnecessary boot of the app, and leads to significantly faster system spec run times.

In local testing, this takes the full system-spec suite for chat from ~6min to ~4min.

app/controllers/session_controller.rb

@@ -113,7 +113,12 @@ class SessionController < ApplicationController
       raise "User #{params[:session_id]} not found" if user.blank?
 
       log_on_user(user)
-      redirect_to path("/")
+
+      if params[:redirect] == "false"
+        render plain: "Signed in to #{params[:session_id]} successfully"
+      else
+        redirect_to path("/")
+      end
     end
   end