Add integration tests for db2json - json2db

With db2json and json2db as the first script entrypoints we can now start with small integration tests. A low-hanging fruit would be to use db2json to convert all sync databases in a container to the respective set of JSON files and then use json2db to re-create the databases again (in a temporary location).

Ideally we would then make use of pacman's DBPath (see man 5 pacman.conf) to test whether pacman plays nice with the created dbs (e.g. by searching for files and/or trying to install things).

Using pytest's mark, we can specifically mark the described tests as integration.