Spawning off of #1141.

The fixture snapshots are pretty much impossible to read, and them being in one file makes the diff really hard to understand (which snapshot is which file?).

Also the snapshot format (jest's default object serialiser) makes it harder to read by adding in Object and Array everywhere.

Need to refactor all of this so that:

• each fixture test is output its own snapshot file
• the snapshot formatter outputs as pure JSON