s1gw: f_ConnHdlr_pfcp_assoc_handler(): handle already-connected case
Previously, f_pfcp_assoc() would skip spawning a connection handler entirely if the IUT was already PFCP-associated. This was fine for tests that only need an association to be present, but upcoming PFCP Heartbeat test cases need the emulation's Recovery Timestamp to be synced with what the IUT has stored, even when re-using an existing association.
Move the state check into f_ConnHdlr_pfcp_assoc_handler(): if the IUT is already connected, sync PFCPEM's Recovery Timestamp to the value the IUT remembers (rrts); otherwise perform the full Association Setup as before. Remove the now-redundant g_pfcp_assoc component variable from test_CT - it's not accessible by ConnHdlr(s) anyway.
