Improve test coverage from 70%

[ludwigschubert]

~61%~~64%~70%

Remaining:

lucid/optvis/transform.py                        74     57    23%
lucid/optvis/objectives.py                      203    133    34%
lucid/optvis/param/lowres.py                     23     12    48%
lucid/misc/environment.py                        14      7    50%
lucid/misc/io/showing.py                         47     17    64%
lucid/optvis/render.py                           90     22    76%
lucid/optvis/param/spatial.py                    37      8    78%
lucid/misc/io/saving.py                          41      7    83%
lucid/misc/io/serialize_array.py                 47      8    83%
lucid/misc/io/reading.py                         58      9    84%
lucid/optvis/param/resize_bilinear_nd.py         37      6    84%
lucid/misc/channel_reducer.py                    26      3    88%
lucid/optvis/param/images.py                     17      2    88%
lucid/misc/io/writing.py                         29      3    90%
lucid/optvis/param/color.py                      21      2    90%
lucid/misc/gradient_override.py                  36      3    92%
lucid/modelzoo/vision_base.py                    30      1    97%
lucid/misc/io/loading.py                         46      1    98%

[ludwigschubert]

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