Make sure that disks unmount successfully before proceeding

[fwcd]

While macOS seems to be able to happily mount images on top of each other, we should probably take care of this:

$ df -h
...
/dev/disk7s1    145Mi  133Mi   23Mi    86%    2686  4294964593    0%   /Users/<user>/git/mixxx-regression-finder/mnt
/dev/disk15s1   145Mi  133Mi   23Mi    86%    2685  4294964594    0%   /Users/<user>/git/mixxx-regression-finder/mnt
/dev/disk16s1   156Mi  133Mi   25Mi    85%    2806  4294964473    0%   /Users/<user>/git/mixxx-regression-finder/mnt
/dev/disk17s1   158Mi  133Mi   25Mi    85%    2821  4294964458    0%   /Users/<user>/git/mixxx-regression-finder/mnt

The reason for why these couldn't be unmounted in the first place was possibly due to mixxxdj/mixxx#11516.