[ENH] Make TEST_SERVER_URL configurable via environment variable

[Sandipmandal25]

Metadata

• Reference Issue: Fixes[MNT] Failures when running tests against locally replicated test-server #1657
• New Tests Added: No
• Documentation Updated: No
• Change Log Entry: "Make TEST_SERVER_URL configurable via OPENML_TEST_SERVER_URL environment variable"

Details

Running tests against a local server currently requires editing openml/config.py directly — fragile and CI-unfriendly. This PR introduces OPENML_TEST_SERVER_URL so developers can point tests elsewhere without touching source code:

export OPENML_TEST_SERVER_URL=http://localhost:8000
pytest -m "not production_server"

Follows the existing pattern of OPENML_CACHE_DIR and OPENML_TEST_SERVER_ADMIN_KEY. No behavior change when unset — defaults to https://test.openml.org as before.

This is also the approach requested in #1629 (review by @geetu040 ) to avoid modifying source code for CI-based local server testing.

[Copilot]

Pull request overview

This PR makes the test server URL configurable via the OPENML_TEST_SERVER_URL environment variable, enabling developers to run tests against local or custom test servers without modifying source code. This addresses issue #1657 about failures when running tests against locally replicated test servers and supports the dockerized CI testing workflow from PR #1629.

Changes:

• Adds OPENML_TEST_SERVER_URL_ENV_VAR constant following existing naming conventions
• Makes TEST_SERVER_URL read from environment variable with fallback to default https://test.openml.org

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[geetu040]

I am not sure if we need this in the long-term since we expect multiple API versions in future. But let's keep this PR open until we reach a conclusion.