I added a simple test for the servent module. All it does is check that the function prototypes of getservbyname and getservbyport are correct and that the functions can be called. Therefore it will fail if $(SERVENT_LIB) is set incorrectly.
POSIX says [1]: This information [serivces] is considered to be stored in a database that can be accessed sequentially or randomly. The implementation of this database is unspecified. And does not explicitly say if any services are required. Because of that, I did not test the behavior of the functions. Though, in practice I believe every OS (unsure about Windows) uses /etc/services. So maybe it is best to just assume it will work normally and improve the tests. Collin [1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/getservbyname.html