IMO, you should eat the same way before a blood test (except that many you have to fast for 12 hours) as you normally do (including the OGTT if you want to take that test). There's no need to find ways to "fool" the test. You want to know what your real results are so you can be better informed, otherwise why even take the test in the first place?
The important thing is to have a doctor who will help you interpret the results as part of your general health. Not a prescriptive (you had X score so I am going to give you Y medicine).
If it is to determine Gestational Diabetes, I've been advised by multiple midwives to make sure I am eating at least 100g of carbohydrate per day in the week leading up to it.
The parameters that your healthcare provider has been given to interpret the test are based on someone eating the typical carbohydrate based diet. Being in ketosis will screw with the results.
Going in there VLC, you are in a state of functional insulin resistance, and the test can't differentiate between that and pathological insulin resistance and glucose intolerance.
Eating a solid meal of mixed macros is important the night before the test as well. I know someone who bombed the test (but it turned out did not have GD) because she went out for hot chocolate the night before the fast and forgot to have a big dinner.