test/bootchooser: extend to cover custom backend
test/bootchooser: extend to cover custom backend

Signed-off-by: Christian Bräuner Sørensen <e4f19a314d4a07f075cfb70502e8c06fa33b6021@prevas.dk>

rauc/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc