Test schema_version_check directly

What does this MR do?

Un-quarantines spec/tasks/migrate/schema_check_rake_spec.rb

Changes spec to invoke schema_version_check instead. Testing via db:migrate causes side-effects like setting ActiveRecord::Base.configurations and also it runs schema dump into db/schema.rb. Rather than keep up, we test schema_version_check directly

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63913

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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