Currently, I am signed up for physics and US history this Saturday. I want to drop the history test. Will it still show up on CB that I signed up for it? And if colleges know that I originally was going to take it, will they care that in the end I did not? I know subject tests are only a tiny part of your application .
No, it will not appear anywhere. You will simply forfeit the $12, or whatever the additional fee was for that exam.
Correct. In addition, colleges will not care if you dropped it, assuming that you still have however many subject tests, if any, that they request.