Test gauge plug on domestic gas system
You are typically going to have a sediment trap required by code that has a nipple which could probably serve both purposes.  The sediment trap is required by code.

http://www.inspect2code.com/residential-code-requirement-for-sediment-trap/

In order to pressurize your gas lines you'd need to disconnect from your gas meter so having the nipple outlet after your house is hooked to the gas meter doesn't seem like something that would be a requirement.  If after you've hooked to your gas line and modifications are made and you need to pressure test again you'd probably just disconnect from the meter and mount the pressure testing device at that location but again you'd have to go to each of your gas appliances and ensure everything is capped/shut off.

Plumbing works the same way, you pressurize your water supply pipes with air or water before any is connected up to fixtures but there is no requirement to keep some kind of access area where you could do this after it has passed this stage of testing.