backend: add test case for last_modified
backend: add test case for last_modified

Make sure that the last_modified value is updated when a prefix is
edited.

SoundGoof/NIPAP,bbaja42/NIPAP,fredsod/NIPAP,plajjan/NIPAP,garberg/NIPAP,ettrig/NIPAP,garberg/NIPAP,SoundGoof/NIPAP,ettrig/NIPAP,bbaja42/NIPAP,SoundGoof/NIPAP,SoundGoof/NIPAP,bbaja42/NIPAP,plajjan/NIPAP,garberg/NIPAP,garberg/NIPAP,bbaja42/NIPAP,fredsod/NIPAP,fredsod/NIPAP,fredsod/NIPAP,bbaja42/NIPAP,garberg/NIPAP,plajjan/NIPAP,ettrig/NIPAP,bbaja42/NIPAP,ettrig/NIPAP,garberg/NIPAP,SpriteLink/NIPAP,SpriteLink/NIPAP,SpriteLink/NIPAP,plajjan/NIPAP,SoundGoof/NIPAP,SoundGoof/NIPAP,plajjan/NIPAP,SpriteLink/NIPAP,fredsod/NIPAP,plajjan/NIPAP,ettrig/NIPAP,SpriteLink/NIPAP,fredsod/NIPAP,SpriteLink/NIPAP,ettrig/NIPAP