This arose from [discussion]:

CI is currently running tests serially in a deterministic order, with some order dependencies due to lack of isolation of both the default connection & test files.

This PR improves the tests by:

• Randomizing their order: to surface any order dependencies
• Enabling parallel tests: to reduce total test time
• Using unique path fixtures or table names to eliminate conflicts across concurrent tests
• Setting up infrastructure to perform more threading tests, whether in GIL or no-GIL workflows.

Workflow Addition

• additional_testing.yml: Additional workflow that performs random, multiprocess, and threading tests.

Configuration Changes

• Add four pytest plugins: pytest-randomly, pytest-xdist, pytest-run-parallel and pytest-timestamper
• --quiet the uv export - the CI output was excessively long and unreadable
• Add a 5 min timeout for individual tests, to avoid test hanging in CI / improve detection of which test hung

Test Changes:

• Modify the duckdb_cursor fixture (conftest.py) to use a tmp_path
• Modify tests creating / deleting files to use tmp_path fixtures, to ensure test isolation
• Fix slow test_query_interrupt - was taking 1 minute per run, and was wrong (I believe)
• Move slow 10M row test_relation to tests/slow

Pytest Plugins

• pytest-randomly: Applied by default. Randomizes order of all fixtures. Found some test issues already.
• pytest-timestamper: Adds timestamps to the --verbose output.
• pytest-xdist: When -n <N | auto> is passed, uses multiple workers to run each test. Speeds up testing, and identifies new test issues.
• pytest-run-parallel: When --parallel-threads=N --iterations=X is passed, runs multiple threads for each test. Many tests won't work with this, due to concurrent use of default connection.