remove superfluous code from bin/test/unit

• Changing directory at the start of the script is not needed because "yarn test:unit" ensures that the script is called from the project root. You can even run yarn test:unit from a sub-directory and it will still have the correct path.

• docker ps --filter name=signalboost_dispatcher* -aq | xargs docker rm -f fails when the filter finds no containers. However, it's not needed, because the unit tests do not appear to leave stopped containers around. If they did, the name would be start with "signalboost_test_runner_run" not "signalboost_dispatcher."