Fix spec of google oauth2

What does this MR do and why?

Related issue: #3003 (closed)

Dependent Upstream MR: Add prepend_mod on SaasRegistrationHelpers

What

Change default parameter in spec from google_oauth2 to gitlab.

Fix failed job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/7733869

1) Trial flow for user picking company and creating a project registers the user and creates a group and project reaching onboarding
     Failure/Error: super
     Capybara::ElementNotFound:
       Unable to find link "oauth-login-google_oauth2"
       Timeout (30s) reached while running a waiting Capybara finder.
       Consider using a non-waiting finder.
       See https://www.cloudbees.com/blog/faster-rails-tests

Why

Jihu SaaS does not support google_oauth2, while Upstream uses google_oauth2 as the default parameter during testing.

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Before	After

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