[dev] add `server-only` tests

[zetavg]

Related to #363.

Not working since the beforeAll hook will run for every test file and the second test file will fail due to

 FAIL  tests/server-only.test.ts [ tests/server-only.test.ts ]
Error: Port 3111 is already in use, please check if other processes are listening on this port (lsof -i:3111) and stop them, as they might interfere with the tests