But they are somehow fragile as you need a special setup to run them (eg load extensions into pg/mysql/sqlite, configure accounts so that connections will work properly...). So it is not a matter of typing "make check" and everything is transparent...I understand. I am not sure how it is done in similar packages where tests depends on some infrastructure.
I wonder as well. Basically you have to specify how to install and configure softwares or even a VM to do the testing, so it will not be "light weight".
-- Fabien.