In my experience, unless you have a significant strength reserve, like using 32kg or more for long sessions of many repeats, low rep intervals will not get the job done by themselves. At best you will be able to struggle through it.
The low rep intervals can lay a good foundation, but (also in my experience) you need specific training with extended sets. For instance, the set/rep scheme that I and a lot of others prefer is 20/20/15/15/10/10/10 (although most people split the last 10 into 5/5, I prefer to start and finish on my dominant hand with one fewer hand switch). To make this work, sets of 20 have to be very comfortable, and that is not going to come from practicing with only sets of 5. Many others go with shorter sets, such as 10/10 throughout, but you are still working continuously for 4-5 minutes (I also strongly recommend being able to complete the test without putting the bell down -- it's possible to do it with breaks, but I find that the time and effort of parking the bell and restarting from a dead stop is not worth the amount of rest you get).
Specific practice with extended sets and practicing the test itself will reveal your weaknesses (Grip strength/endurance? Hip power/endurance? Running out of breath?) and allow you to adjust your training accordingly.
There's also the mental comfort level of practicing the test regularly so you have confidence you can do it, you get a feel for the pacing and set/rep scheme that work for you, and you can stay relaxed despite fatigue or discomfort.
Finally, if your lockout position does not allow you to comfortably support the bell with your structure and alignment, and you have to fight too much to keep the bell overhead with muscle power, you accumulate fatigue much faster, and this is something you don't necessarily notice when only doing short sets. If your lockout position is restricted, an ounce of better mobility can be worth a pound of strength and stamina.
Good luck. You have plenty of time to make sure you have the test squared away, and doing so will remove a lot of stress from the cert and make it easier to focus on learning.
