In order to run meaningful tests on the core-avr libraries, we build x86 binaries to [project name]/build/Test.exe. These executables each run test suites.
In order to build, you'll need to download the catch.hpp single include and provide the path in properties.mk. This path will be included as a -I include for all projects.
Each project has a test directory that new suites may be added to. If a new suite is added, simply add the new file to the makefile file, under the SOURCES property.