From my own experience, when the instructors nominate you for a test you are already considered to be at that particular level. Most of the "evaluation" is done during regular training. So, clear your mind, focus on the moment, perform each technique as slowly as necessary to make it clear, and above all enjoy it. To illustrate the importance of basics and principles, consider the following total requirement for promotion to
Sandan in ASU:
"All basic techniques performed to demonstrate more understanding of the relationship of basic principle to the technique with maturity and clarity." And that's the updated version; for years it was simply "All previous techniques performed demonstrating kokyu-tanden-ho".
I've been there. Before returning to active duty I was in the National Guard, and it seemed that every test date coincided with a weekend drill
Needless to say, it only got worse when I returned to active duty. It took me almost eight years to test for Shodan, but then again I had a great test.
Looking back, I have never noticed any dramatic improvement in my technique the day following any promotion, much to my chagrin
The truest test lies in finding a way to deal with a committed and sincere attack, in any case.