You can use Free5GC's set of testcases (free5gc/test/registration_test.go) to test other core networks, too.
1.) change IPs in registration_test.go to point to your 5GC instance.
const ranN2Ipv4Addr string = "127.0.0.1"
const amfN2Ipv4Addr string = "127.0.0.1"2.) run tests with the noinit parameter -- this will inhibit initialization of a local free5gc instance
~/free5gc/test$ go test -v -run TestRegistration -args noinit