tests: convert IPA BTS example to use event-oriented approach
tests: convert IPA BTS example to use event-oriented approach

Like the HSL BTS example. We use an event file descriptor
which is used to call the GSM 12.21 based function for
handling.

osmobuntu/libosmo-abis,osmobuntu/libosmo-abis,osmobuntu/libosmo-abis,telenoobie/libosmo-abis,osmobuntu/libosmo-abis,telenoobie/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis