If I recall correctly, for testing you do not need to upload a build to iTunes.
However (and this is very important) when you are ready to release your app and you submit it to Apple, you will need to submit your IAP products at the same time. They need to be approved, just as the app itself does.
To submit them, on the app details page on iTunes Connect there should be an IAP section near the bottom (below your screenshots etc). Just tick all the IAP you want to submit for review, and then submit your app when ready. I think the same applies to any Game Centre achievements etc, but it’s been a while so I could be wrong about that.
If you fail to submit the IAP they will not appear in your released app, so make sure you do it. If you wanted to play it safe, you could submit a test build early and submit the IAP at the same time, but make sure to set the Version Release to “Manually release this version” so that it does not go live. You only need to get IAP approved once (assuming you don’t change them), so they’ll be good to go from that point. Then just resubmit the build itself when you are ready to release.