Use both pytest-split and pytest-xdist for parallel CPU test execution

[shralex]

Previously, CPU tests are distributed across multiple runners using pytest-split, which assigns the same number of tests to each worker. However, since the runtime of tests is different and each runner gets few tests, some workers end up finishing fast and stand idle while others take a long time, so we're not utilizing the workers fully. In addition, only one process is used within each runner, despite many CPUs available.

In this PR we're still statically splitting the CPU tests across multiple runners using pytest-split, but we're reducing the number to 2 instead of 4 (so each runner gets more tests), and now also utilizing multiple concurrent processes and dynamically assigning work to these processes within each runner using pytest-xdist.

Overall, we're reducing the number of runners from 4 to 2, the runners are now well balanced and finish approximately at the same time (since each gets assigned more tests) and slightly reduce the total CPU test run time by ~20sec.

If CPU tests become a bottleneck just add another runner to the matrix.

Tests

Testing in CI.

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