Pass bool instead of int values to pytest.mark.skipif

[obi1kenobi]

Resolves the following type errors that mypy points out:

arviz/tests/base_tests/test_utils_numba.py:15: error: Argument 1 to "__call__" of "_SkipifMarkDecorator" has incompatible type "int"; expected "Union[str, bool]"  [arg-type]
arviz/tests/base_tests/test_plot_utils.py:203: error: Argument 1 to "__call__" of "_SkipifMarkDecorator" has incompatible type "int"; expected "Union[str, bool]"  [arg-type]
arviz/tests/base_tests/test_plot_utils.py:281: error: Argument 1 to "__call__" of "_SkipifMarkDecorator" has incompatible type "int"; expected "Union[str, bool]"  [arg-type]
arviz/tests/base_tests/test_diagnostics_numba.py:17: error: Argument 1 to "__call__" of "_SkipifMarkDecorator" has incompatible type "int"; expected "Union[str, bool]"  [arg-type]
arviz/tests/base_tests/test_stats_numba.py:18: error: Argument 1 to "__call__" of "_SkipifMarkDecorator" has incompatible type "int"; expected "Union[str, bool]"  [arg-type]

[obi1kenobi]

I just moved this to the top (it used to be in the middle of the file) and then made all the skipif decorators in this file use the same logical expression as the one on line 310. I chose the expression from that line since it seemed the most obvious and explicit to me, but I'm happy to switch to a different one if the maintainers have a different preference.

[ahartikainen]

Thanks for the PR