Improve test coverage

[zerolab]

Name                                   Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------------------------
src/wagtail_bynder/__init__.py            15      5      4      1    58%   20, 24-29
src/wagtail_bynder/blocks.py              15     15      4      0     0%   1-22
src/wagtail_bynder/middleware.py          20      0      8      1    96%   13->31
src/wagtail_bynder/models.py             142     64     20      0    51%   106, 120-129, 132, 135-136, 149-152, 155-156, 183-188, 202-222, 228-239, 255, 318, 322-324, 328-330, 336-366
src/wagtail_bynder/utils.py               44     23      0      0    48%   26-31, 35-44, 48-57, 74-89
src/wagtail_bynder/views/document.py      44      3      4      1    92%   14, 28, 31
src/wagtail_bynder/views/image.py         43      3      4      1    91%   14, 28, 31
src/wagtail_bynder/views/mixins.py        37     12     10      1    60%   11, 34-36, 39-44, 47-52
src/wagtail_bynder/views/video.py         47     12      6      1    75%   15, 30, 35-41, 54-56, 67-68
src/wagtail_bynder/wagtail_hooks.py       29      4     18      3    81%   17->exit, 42->exit, 53-54, 68->exit, 71-72
src/wagtail_bynder/widgets.py             30     30      4      0     0%   1-52
----------------------------------------------------------------------------------
TOTAL                                    479    171     86      9    62%

• Management command updates (various) #10
• Conditionally sync videos when chosen #12