Add tests

[amclain]

There should be a Nerves application in tree that can be used to at least smoke test that the system comes up. The Nerves build-tools build-tools/build-test CI job should also be added. This is also important to see if building the firmware bundle fails at the application layer (mix firmware.burn). This can happen when versions of system components are bumped and the names of the bin files change.

There is an example of this test directory in the official Nerves systems, like nerves_system_bbb.