Female here, did my fitness test a couple weeks ago. My only advice is start now! Just keep practicing and practicing until you can very comfortably do it. You don't want to be pushing yourself to the limit just to scrape through the run for example, to then be absolutely wrecked and out of breath and not be able to do the push ups/sit ups.
As most women generally do, I struggled most with the push ups. But I just kept doing them and doing them, there's no way around it, you just have to do them and get used to doing them. I started doing push-ups on my knees and found that it didn't help with the actual push-ups at all. You need to practice the form of the push up so trying to make it easier by going on your knees might not actually help!
Let me know if you have any other questions about the fitness test! I'm sure if you're going for a PTI role then you should be keen about fitness anyway so should fly through it!