Fix flaky test on CI

What does this MR do and why?

Fix flaky test on CI

When extracting some pages-related methods from Project to Gitlab::Pages::UrlBuilder, the Project#pages_subdomain was removed, therefore the shows the DNS ALIAS record spec on spec/features/projects/pages/user_adds_domain_spec.rb had to be refactored.

This introduced a bug, where the namespace used in the test was hardcoded.

To fix this, I'm reverting the test to use dynamic values based on the variables built for the test.

Related to: gitlab-org/quality/engineering-productivity/master-broken-incidents#3214 (closed)

Changelog: fixed

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