Add "low-level" unit tests

[i-ky]

With source code now separated into several source files (aka units) it now makes sense to add some sort of framework to test these pieces one by one. Such tests will be faster and can be more thorough than full-scale integration tests featuring zabbix_agentd, zabbix_get and dummy Modbus devices. I'm currently looking at µnit.

[v-zhuravlev]

I think that integration tests should stay as well. It will be nice to do things not covered by the already implemented test first of all.

[i-ky]

Sure. "Add", not "replace".