Remove return statement from test

[mdpiper]

The latest version of pytest throws an error if a test returns anything other than None. I've removed a return statement from one test in bmi-tester.

This fixes #50.

[mdpiper]

This PR exposed two unrelated maintenance issues:

1. an outdated pre-commit hook, and
2. use of Mambaforge in the CI test workflow

which I fixed in a pair of commits.

[coveralls]

coverage: 11.97% (+0.02%) from 11.952%
when pulling d60c92b on mdpiper/remove-return-value
into c4f5c99 on master.

[mcflugen]

👍