Changes

ttcn3-bts-test: use start_config functions

As suggested by Pau in code review, refactor the file to use functions
for starting the testsuites, SUT and related containers with specific
configurations.

Related: https://gerrit.osmocom.org/c/docker-playground/+/36389/comment/67a7c35e_766943a3/
Change-Id: If3cb7dbe585c03d21e5ec6353fd25575ccac7af0