Pressure testing radiator system with expansion tank
Check the rating on the expansion tank, but I'm almost certain it would be able to withstand 15-20 PSI.  But I'd still remove it and here's why.  If you have a slow leak, the expansion tank would act as a buffer (add volume), to your system.  This would make any loss of pressure much slower, perhaps to the point where you didn't realize you have a leak.   

Is there a reason you don't want to use water to test?   Water doesn't compress and any leaks would result in rapid pressure drop.   You'd have to control for temperature variations but over a few days, you'd know if you had a leak.