MFT: testport -n: Respect the port's advertised PREFIX if not already set in env or poudriere.conf

This fixes 2 issues.

1) bulk was already respecting the port's PREFIX. testport would report different
   failures than bulk -t would.
2) 75 ports define their own PREFIX. This may be worth changing in the ports.
   While they are doing this, we should test them with their advertised default
   PREFIX.

This change does not impact the default of PREFIX=/prefix/ for testport