I just got all the parts for my new PC (A Ryzen 7 3700x, Strix x570-E motherboard etc.). Unfortunately as I was inspecting the stock cooler, I scuffed away some of the pre-applied paste. I then decided, for some reason, to just apply new thermal paste, so I wiped away the remaining paste. But now, the thermal paste I ordered is stuck in shipping and will take at least a week to reach me.
So my question is: Can I just put together all the parts for 5 minutes, just to see if the build passes POST. I don't plan to install anything etc. Is this dangerous, and will it void my warrantee?
I highly suggest you don't. First your CPU will overheat within seconds after power is applied and turn off the computer to prevent heat damage. It may last a little bit longer then a few seconds if the processor and CPU Cooler heatsink are making a tight contact. But unless you want to take a chance of damaging your new Processor it is up to you.
I suggest you either go to a local Walmart or other store that sells computer hardware and software and see if they sell Thermal Paste or wait for the thermal paste you ordered.
NOTE: It will void your Warranty since you have improperly installed the CPU Cooler without Thermal paste.
There are hard coded safeties which will trigger an instant shutdown if the processor passes the safety threshold, so you won't cause damage, but it won't last 5 minutes unless you put some kind of extra cooling on it, such as a box fan on high blowing directly on it.
Really, especially for short term (a few hours at most), any kind of paste will work as long as it's not watery, as all you're trying to do is eliminate the air gaps. TomsHardware, in their TIM test a few years ago (2013), included denture cream and toothpaste in their lineup just for fun. Their CPU was a Intel Core 2 Quad Q6600, which had a 105w TDP, and they got temps of 67*C under load after an hour of burn in time. They also used toothpaste and came out with a result a few degrees higher. Just make sure you clean it well.