Fix incorrect tests in test_usm_ndarray_dlpack

[ndgrigorian]

The test test_dlpack_device attempted to query a device using DLPack device ID from the list of root devices created by all_root_devices fixture in test_usm_ndarray_dlpack.

This logic is incorrect, as all_root_devices caches only the first two root devices per platform. As the DLPack device ID is based on the ordinal ID of the device as it is canonically presented in get_devices, this could fail if a given platform had more than 2 associated devices (in this case, for a machine with 2 PVC. Each has 2 stacks, and therefore, 4 level_zero and opencl:gpu devices).

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• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
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• Have you added your changes to the changelog?
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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1982/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.19.0dev0=py310h93fe807_503 ran successfully.
Passed: 894
Failed: 2
Skipped: 118

[coveralls]

coverage: 88.24%. remained the same
when pulling 7ef0e44 on fix-dlpack-test
into bfd4d57 on master.

[oleksandr-pavlyk]

Yes, you're right. Expect this change to slow down run-time of this test file on the machine with many cards, but correctness first.

Was it failing in Tiber?