* [AMF] Fix UE context transfer when only NRF is client
If UE context transfer is initiated and the new AMF does not get the
old AMF from NRF or no UE context is retrieved from the old AMF,
we do not want to reject UE registration. Send identity request instead.
Test "transfer-error-case" is added into the commit.
* [tests] Unite tests for UE context transfer
All tests for UE context transfer with different configs are placed
into test folder transfer.
* [tests] Make two binaries for UE context transfer tests
For each test config a different test binary is created.
Previously, sample.yaml was used for both 5GC and EPC attach tests.
Because SMF had SBI configured, it sent a register PUT to NRF even in
EPC-only tests (where nrf/scp was not run), leading to a missing HTTP
response and connection timer expiry.
Now, attach.yaml is used for EPC, preventing the unwanted NRF registration.
