Changes

common/ttcn3-docker-prepare: add _scripts symlink

The ttcn3-*-{start,stop}.sh scripts include a file from the _scripts dir
since Ia63c37b54fb42a8c64a676334cef6f9df4152e18. As they are called
through symlinks in / in docker-playground.git, we must make the another
symlink available for _scripts as well.

Fix for:
  ../ttcn3-tcpdump-start.sh: 3: .: cannot open ../_scripts/tcpdump-dumpcap.inc.sh: No such file

Change-Id: I3a093ac7058d31db958785692444cb58525a6181