Changes

tests: fix for --disable-ortp

osmo_ortp part of libosmotrau is now optional, can be disabled
for systems on which libortp dependency is too onerous.  Fix the
testsuite so it can run when built with --disable-ortp.

Related: OS#6474
Change-Id: I3c59cfab78f58935a5ab6ce7b5f0ed8aadd853a8