You could create it as a Block against the drinker using magic, and would need enough shifts to counter the amount of power you could be compelled to unleash - at least your Conviction.
But, as it is with all things Wizardly, you would benefit from knowing a lot more about the nature of these tests. What will they ask you to do? How long will they be testing you? Preparation is key. If you don't want to RP sessions with other White Council recruits, maybe make some pre-test Assessments that will give you Aspects you can tag for bonuses, such as Informed about the White Council Tests, Gaming the System, How to Hide Magic from the White Council. Edited to add: but if you are the GM, I can see wanting to get some suggestions for guidance.
Options:
Versus Compels: You could add maybe four more shifts in case the White Council/GM tries really hard to Compel your character to do magic, or simply amass enough Fate Points to buy out of anything they try to force you to do.
Duration: How long does it need to last? since it would be a Thaumaturgy effect, it would last for an entire scene, but if the testing is going to take several scenes, you may want to put in additional shifts to enhance the duration. The book assumes a scene to be roughly equal to 15 minutes on the Time Chart, so for a day's worth of effect, you'll want to add 5 shifts.
Potion of Magical Suppression
Ingredient suggestions:
Base Liquid: flat root beer
Touch: a classified ad clipped from a tabloid paper
Taste: a dab of ranch dressing
Smell: potpourri from a psychiatrist's waiting room
Sight: a picture of a landfill
Sound: a piece from a plastic flag from a used car lot
Mind: a sliver of charred wood from an old witch-burning site
Heart: a used lottery ticket
