Add first unittest

[johnny9]

This adds an initial unittest against the bitcoinamount module. This is the simplest c++ module to test so its an easy first target. I think the Qt test framework is the best fit for the project and eventually I imagine GMock will be needed to mock out portions of bitcoin interfaces as well as more complex modules get tested. These changes update the CMake definitions and add a BUILD_APP_TESTS option to toggle building of the unittests.

A new ci workflow is added as well to run the unittests (and eventually other testing that we have).