Fix bz_plot testing
Fixes various testing issues with the new bz_plot
functionality. Closes #1472 (closed).
Checklist
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I am familiar with ASE's contribution guidelines. -
Doc strings in code changed in this MR are up to date. -
Unit tests have been added for new or changed code. -
Issue is resolved via "closes #XXXX" if applicable.
Edited by Andrew Rosen