[MNT] Intermediate test plan

[geetu040]

We want to move tests from relying on the remote server (test.openml.org) to a locally running server setup. This is needed to avoid race conditions and issues caused by a shared, mutable central database (as discussed in #1586).

At the moment, test cases use both the production server (www.openml.org) and the remote test server (test.openml.org). The goal is not to remove all remote calls at once, but to gradually replace calls to test.openml.org with calls to a local server.

The local setup will be started via Docker containers in the CI workflows. We will need three services:

• test-database
• php-api-v1
• python-api-v2

The SDK does not yet support python-api-v2; work on this is ongoing in #1575. However, we still plan to enable the local python-api-v2 service now, since tests targeting it will be added as the migration progresses.

I'll follow up with more details on:

• how these Docker services will be started programmatically in the workflows, and
• how the pytest configuration can be adjusted to use the local services instead of remote endpoints.