Introduces new unit tests based on `check`. Each subsystem/module can
have its own unit tests in tests/check-<name>.c that will be linked
together in tests/check-all.c.
Running 'make check' will compile and run the unit tests automatically.
A reference unit test implementation has been done for the abstract
address module.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
