Move tensor tests from dpctl

[vlad-perevezentsev]

This PR moves all tensor-related tests to dpctl_ext/tests/tensor as part of the ongoing migration of tensor functionality from dpctl to the new dpctl_ext package.

Key changes:

• Relocated 89 tensor tests (elementwise functions, usm_ndarray, and tensor utilities)
• Updated imports to use dpctl_ext.tensor
• Added test infrastructure for proper discovery (dpctl_ext/tests with conftest.py)
• Included tests in packaging configuration
• Integrated tensor tests into CI
• Fixed several issues discovered during migration (dtype expectations, boolean reductions, etc.)

In a follow-up PR:

• Conditional logic will be added to run dpctl_ext/tests only when changes affect the tensor code.
• Array API tests for tensor will be introduced and executed as a separate CI job.

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• 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?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2817/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev3=py313h509198e_279 ran successfully.
Passed: 1355
Failed: 2
Skipped: 16