Fix flaky test: Signup on EE for SaaS allows visiting of a page after initial registration

Summary

This MR fixes a flaky test that was failing with database lock timeout errors during organization creation.

Test Details:

• File: ee/spec/features/signup_spec.rb
• Description: Signup on EE for SaaS allows visiting of a page after initial registration
• Issue: https://gitlab.com/gitlab-org/quality/test-failure-issues/-/issues/7295
• Flakiness: 29 reports (flakiness::3 severity)