Add blurhash autotests

It currently tests decoding, and the new URL decoding mechanism. There's two images, one for a common hash (percent-encoded and without) tested through the blurhash.h decode function and the ImageProvider as well. There's also a troublesome hash I found, that has a special %14 case that trips up the blurhash provider before I fixed it.

I also caught a bug in the original MR (!108 (merged)) that was missing a certain encoding character, so that's also included in this patch.