I have several Pis and two Arduinos, all of my sensors are on breadboards.
We may be thinking of two different definitions of testing.
1) Static testing of code (sensors and infrastructure) during development.
2) Dynamic testing of the system while it is running
I have been thinking of the first, while I think you more of the second.
The software and Pi don't know (or care) about whether the sensors are wired through a breadboard, or attached via a custom shield; hence I can get by running pieces of the Food Computer, without actually having a Food Computer.
This is getting off topic, but if you just want to grow a head of lettuce .....