tests: only check IU configs if IU is enabled

Fix vty tests that are failing since libosmocore change
Ic225232fbfca49ba868427eaf898e1f6e34e1ca8. If OsmoMSC is built without
IU support, it fails with "cs7-instance-iu" in the config.

Change-Id: Ie56da9167badfd2399b566af91a345103f46c2a1
2 files changed