This makes the tests also work on systems which don't run X.

See ChangeLog / man/feh.1 "--thumb-redraw" for more

Depending on --thumb-width/--thumb-height, either .thumbnails/normal or
.thumbnails/large is used. For higher dimensions, thumbnail caching is still
silently disabled, this should also be what the standard indicates.

(This was actually meant to be included in
f4420709)

Centering didn't work if the text below the thumbnail was wider than the
thumbnail itself. Fixed.

The ButtonRelease check only knew pan mode, so releasing the blur key changed
to the next image.

I know that there is one in the old SVN repo, but I think a rewrite
(with an automated test script) is due.