Test local installations

[gselzer]

As I've now found out with both d9fc267 and #307, napari-imagej seems to work poorly with local installations. It would be great to regularly test with a local installation, maybe on CI?

@ctrueden what do think would be easiest here? We could write a script that downloads a Fiji installation, however that seems overkill. Could we do something easier via jgo, making use of its cache?

[ctrueden]

I think we'll need to download and cache a Fiji installation; jgo will not help here.

Here is how PyImageJ does it.

Unfortunately, it looks like we are not caching it, so the CI downloads it every time. 😢 But caching it is a little tricky; we would still want to bring it fully up-to-date before testing each time. Somewhere there is code that does this on CI, but I can't remember where off the top of my head...