Fix `db:test:prepare` with multiple databases

What does this MR do and why?

Fix db:test:prepare with multiple databases

db:test:prepare calls db:test:load_schema. If we define the same database for different database configurations, skip loading schema if we have already done so.

db:test:prepare is called from:

• Internally by Rails, system("bin/rails db:test:prepare") as part if its automatic synchronization of db/structure.sql and the test database.
• GDK (https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/lib/gdk/gitlab_rails/db.rb#L14)

Related issue: #349589 (closed)

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