Changes

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