Add default exclusion of slow tests in pytest

[vlad-perevezentsev]

This PR suggests to exclude running slow tests (marked as slow) when pytest is called by default and to register custom markers such as slow and multi_gpu.

These changes will help to avoid spontaneous hangs in internal CI since some tests are decorated by @testing.slow which is not implemented yet.
It is assumed that such tests (usually using huge data) should perform computation on multiple devices to reduce the computation load. Internal CI contains one device and running such tests causes spontaneous hangs.

To run all tests including those marked as slow (@testing.slow ) use pytest -m "not slow or slow"

$ pytest tests/third_party/cupy/math_tests/test_sumprod.py::TestSumprod
collected 29 items / 1 deselected / 28 selected tests/third_party/cupy/math_tests/test_sumprod.py ...........................s ========================= 27 passed, 1 skipped, 1 deselected in 1.96s=========================

$ pytest -m "not slow or slow" tests/third_party/cupy/math_tests/test_sumprod.py::TestSumprod
collected 29 items tests/third_party/cupy/math_tests/test_sumprod.py ............................s =========================28 passed, 1 skipped in 3.77s=========================

Also removes test skipping for dpnp.power() due to using the latest OneAPI compiler for coverage GitHub Action #1777

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[github-actions]

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

[vlad-perevezentsev]

about my last commit
@antonwolfy do we need to keep using -p no:warnings for addopts?
Does it make sense to add filterwarnings section and handle warnings in it?

[antonwolfy]

Thank you @vlad-perevezentsev, the change is very helpful and makes the tests more stable.
The filterwarnings marker can be added separately.

[vlad-perevezentsev]

Thank you @vlad-perevezentsev, the change is very helpful and makes the tests more stable. The filterwarnings marker can be added separately.

I will keep filterwarnings registration in markers for this PR cause running tests does not work without it.
I will remove it in the next PR along with removing -p no:warnings and add warning handling