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).
