Move the git fetch/checkout code and make call to build the testsuite,
to debian-stretch-titan/ttcn3-docker-prepare.sh. In the next patch, I
will extend the script to update deps right before building too (e.g.
because OSMO_TTCN3_BRANCH changed).
Related: OS#5017
Change-Id: I4b5bedf058dc527e821f9b7204c632820e671af9