Mostly empty unit tests -- added mgos_mock to be able to run unit tests on x86_64

unittest/README.md

@@ -0,0 +1,11 @@
+# Unit tests MGOS Application
+
+This directory contains some small unit tests for the `screen` and `widget`
+libraries. It is meant to compile and run on the host operating system,
+in the author's case this is Ubuntu LTS.
+
+To run the test suite:
+```sh
+make
+./test
+```