Add test with 0d out array for dpnp.einsum

[antonwolfy]

The PR extends tests coverage for dpnp.einsum and align tests names.

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[github-actions]

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

[coveralls]

Coverage is 78.11% — add-0d-out-test-to-einsum into master. No base build found for master.

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev2=py313h509198e_14 ran successfully.
Passed: 1371
Failed: 4
Skipped: 5