I've had those tests many times and had to take the horrible 'Picolax' laxative and whilst I had to fast with clear liquids(This was before diagnosis) the paperwork included a pre-test menu for diabetics which included what they called 'White foods' which meant the day before the test you could eat boiled eggs,white toast,cooked chicken,clear soups,jelly sweets and normal jelly-not great but it meant at least you didn't have to fast being a diabetic.
On a lighter note they told me I could still have a couple of Vodka/Gin and tonics the day before as they were clear fluids and wouldn't affect the test! :lol:
But seriously though,when it comes to the laxative make sure to drink loads of fluids as I got VERY dehydrated due to the amount of loose motions that the Picolax provokes and wet toilet wipes are a godsend too!