Add third-party test for cumsum with axis=None and out

[antonwolfy]

This PR adds test_cumsum_no_axis to the third-party TestCumsum suite in dpnp/tests/third_party/cupy/math_tests/test_sumprod.py aligning with CuPy test suite.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/3088/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev13=np2py314h8d9cdd5_2 ran successfully.
Passed: 1376
Failed: 0
Skipped: 6

[coveralls]

coverage: 78.667%. remained the same — add-cumsum-no-axis-out-test into master