get rid of the sources of warnings in tests

[aaronspring]

You are encouraged to add information about all relevant sections, but not required to submit an issue.

Code Sample, a copy-pastable example if possible

• just fix them
• see Warnings in the test suite pydata/xarray#3266 they have a few good suggestions

Problem description

This should explain why the current behaviour is a problem and why the expected output is a better solution.

Expected Output

Output of climpred.show_versions()

# Paste the output here xr.show_versions() here

[aaronspring]

gazillions of warnings are back

[bradyrx]

I feel like we are always chasing this. But good job catching it. Will also need to think about this for all the docs warnings.

[bradyrx]

Addressed in #441. Let me know if that's true..