[ci] add vllm_test_utils

[youkaichao]

some test scripts cannot import vllm, but they need to be reused.

we only install it in test environments.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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[youkaichao]

there are also some functions repeated in the tests, like distributed_run:

vllm/tests/distributed/test_shm_broadcast.py

Line 21 in 519e8e4

def distributed_run(fn, world_size):

I'd like to gather them into vllm_test_utils so that #10502 can use it.

[DarkLight1337]

This looks reasonable to me.