Running tests with `DPNP_TEST_ALL_INT_TYPES=` raised an exception

[antonwolfy]

The bellow command to run dpnp tests causes the issue:

$ DPNP_TEST_ALL_INT_TYPES= python -m pytest -ra -x --pyargs dpnp.tests
...
/localdisk/work/antonvol/soft/miniforge3/envs/test_issue/lib/python3.12/site-packages/dpnp/tests/test_absolute.py:7: in <module>
    from .helper import (
/localdisk/work/antonvol/soft/miniforge3/envs/test_issue/lib/python3.12/site-packages/dpnp/tests/helper.py:10: in <module>
    from . import config
/localdisk/work/antonvol/soft/miniforge3/envs/test_issue/lib/python3.12/site-packages/dpnp/tests/config.py:3: in <module>
    all_int_types = int(os.getenv("DPNP_TEST_ALL_INT_TYPES", 0))
E   ValueError: invalid literal for int() with base 10: ''
...

It seems the way how env variable DPNP_TEST_ALL_INT_TYPES is handled in tests needs to be corrected.