Fix pytest cleanup for V1 tests

[joerunde]

We currently require using pytest-forked (the --forked CLI arg) for our test runs that include the v1 marker. Something is not being cleaned up properly after each test when using the V1 engine, which causes subsequent tests to fail.

If we can fix the cleanup, then we can remove --forked and have faster test runs without the per-test process isolation

[sducouedic]

Is it likely more an issue with the tests themself (like the fixtures of the tests configuration) or the code in VLLM?