better warning filter for assert_*

[mathause]

In #4864 I added a a decorator for the xarray.testing.assert_* functions to ensure warnings that were to errors (pytest.mark.filterwarnings("error")) do not error in assert_* (see https://github.com/pydata/xarray/pull/4760#issuecomment-774101639). As a solution I added

https://github.com/pydata/xarray/blob/5470d933452d88deb17cc9294a164c4a03f55dec/xarray/testing.py#L32

However, this is sub-optimal because this now removes all ignore filters! As dask stuff only gets evaluated in assert_* filters like warnings.filterwarnings("ignore", "Mean of empty slice") don't work for dask arrays!

I thought of setting

warnings.simplefilter("ignore")

but this could suppress warnings we want to keep.

So now I remove all "error" warning filters and keep the rest. Note that the original filters get restored after with warnings.catch_warnings():. ().

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I am not sure I expressed myself very clearly... let me know and I can try again. @keewis you had a look at #4864 maybe you can review this PR as well?