E2E test: malformed MR url relative-url fix

What does this MR do and why?

Resolves #483173

This test was recently removed from quarantine. It fails in the relative-url job since the /relative path is getting stripped. This results in a 404 rather than the 400 we want.

This MR ensures the path is added onto the address before sending the request.

MR acceptance checklist

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How to set up and validate locally

QA_GITLAB_URL=https://gdk.test:3333 bundle exec rspec qa/specs/features/api/3_create/merge_request/view_merge_requests_spec.rb

QA_LOG_LEVEL=debug QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/api/3_create/merge_request/view_merge_requests_spec.rb