Add spacebar-fakeserver to mock ModemManager

This MR introduces a test program that mocks the ModemManager API in order to test sending Spacebar without a modem.

It currently only supports simple sending and receiving messages, but is useful enough to debug simple issues. No MMS support yet though, maybe at a later date?

You can run this with sudo spacebar-fakeserver (ModemManager also needs to be off when this is the case)