Changes

library/PFCP_Emulation: add f_PFCPEM_set_recovery_timestamp()

Companion setter for the existing f_PFCPEM_get_recovery_timestamp().
A follow-up patch uses this to inject a specific Recovery Timestamp
value into the emulation, so the IUT does not detect a spurious PFCP
peer restart between test cases.

Change-Id: If47e06d26b42081a28aacb6e06b5aeabf945362c