Add test skipna for correlation metrcs

[raybellwaves]

AFAICT

There isn't a test in the correlation metrics for nan.

xskillscore/xskillscore/tests/test_deterministic.py

Line 88 in da06b06

def test_correlation_metrics_xr(a, b, dim, weight_bool, weights, metrics):

There is something a loose nan test test_pearson_r_p_value_skipna

xskillscore/xskillscore/tests/test_deterministic.py

Line 260 in da06b06

def test_pearson_r_p_value_skipna(a, b_nan):

but it just checks it won't yield all NaNs