Delay after fabrication in load balancer tests

What does this MR do and why?

Adds a 5 second delay after user create/delete/update via the API in load balancer tests on GDK

This is a workaround to avoid race conditions that are more likely in load balancer tests when switching users immediately after they're modified via the API (which is not a likely real use case). See #415648 (comment 1448565881)

How to set up and validate locally

From the qa directory:

bundle install
export WEBDRIVER_HEADLESS=false # If you'd like to watch the test in action
export QA_GITLAB_URL="http://gdk.test:3000" # Only needed if GDK is not running on http://127.0.0.1:3000
bundle exec rspec qa/specs/features/browser_ui/1_manage/login/register_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.