How can Arduino source code be tested automatically (continuous integration)?
Newest version of Arduino ide has a command line interface to build and upload code. But you can obviously do it by makefile and avrdude.
Now, you have compiled your code BUT you need testing.
As simulator are complex,incomplete, expansive and.. Just a simulation, and because the chip are relatively cheap, building a board witch will make HW interaction AND check the results its the fastest and easiest way.
On that "special" board you may upload something like an interpreter witch take a test from the PC and execute it, something similar to firmata for arduino.
At least that is how I would build it.
And as far as I know there are no know implementation of this, even if I'm quite sure many industries should and probably do that.