Changes

bts-oml: Set mp_pcu_socket

Since recently (osmo-ttcn3-hacks.git
I78880098a55d1cb456011746efa0a47832a86ca8), BTS_Tests_OML use
mp_pcu_socket to set up PCUIF, which is used in some tests (like
validating forwarding of PCUIF messages PCU<->BSC).

Related: SYS#5303
Change-Id: Iac4aac8d98e2f45021858ddaa5e20ca4c6effb31