Concurrency testing

[paultiq]

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
• misc: move one slow test case to tests/slow, and rework test_query_interruption.

Added Plugins

pytest-xdist

Disabled by default. This PR adds -n 2 to packaging_wheels to run two parallel tests at a time.

Comments:

• GH Runners (often) have 4 cores, and I've had luck with -n auto to let xdist use the number of processors
• Since DuckDB will parallelize the work, this will increase CPU contention, especially for performance intensive tests. There may be some tests where parallelizing is not desired - for those, the solution is to run tests in two steps - once with xdist once without.

pytest-randomly

Enabled by default. Randomizes the order of tests.

Comments:

• Random order is good for surfacing test dependencies and assumptions
• This identified a number of test dependencies, especially around use of a "dirty" default connection or reusing file names