As others have posted above,
Dry Nitrogen is the conventional Way to pressure test, and the EPA states in writing that “small amounts” of Freon can be added to the nitrogen gas for leak testing.

If it is an old R-12 system, some pressure test with
Difluoroethane, the relatively cheap gas in some “Dust-Off” or “Canned Air” spray cans.

In the AC world Difluoroethane is known also as HFC-152A
It is flamable.

You need to buy a “Can Tap” to transfer the gas from the Dust-Off cans through a hose to AC systems.

Some R-12 system conversion details using HFC-152A are here:

https://www.s10forum.com/threads/how-to-r152a-conversion.493953/