ttcn3: Use osmocom registry to pull ttcn3-bts-test

Since recently, the ttcn3-bts-test image is available in the osmocom
registry. Let's use that one instead of the sysmocom one.

The imaqge is built and pushed to the registry regularly by osmocom
jenkins job registry-rebuild-upload-ttcn3-bts-tests (see
osmo-ci.git/jobs/registry-rebuild-upload-titan.yml).

Related: SYS#5765
Change-Id: I6de9719830290aa7f3ca80a6080ed37f107bdc16

sysmocom/ttcn3/jenkins-run.sh

@@ -25,7 +25,7 @@ prepare_docker() {
         PWD=$OLDPWD
 }
 
-docker pull registry.sysmocom.de/ttcn3-bts-test
+docker pull registry.osmocom.org/ttcn3-bts-test
 
 # remove older trial dirs and *-run.tgz, if any
 trial_dir_prefix="trial-"