Remove cmdtest if installed before installing yarn

since both packages conflict,
see yarnpkg/yarn#2821

Note: Since Ubuntu 17.04, cmdtest is installed by default.

musicbrainz-dockerfile/Dockerfile

@@ -31,6 +31,7 @@ RUN cd musicbrainz-server && \
     apt-get update -o Dir::Etc::sourcelist="sources.list.d/yarn.list" -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0" && \
     curl -sLO https://deb.nodesource.com/node_8.x/pool/main/n/nodejs/nodejs_8.11.3-1nodesource1_amd64.deb && \
     dpkg -i nodejs_8.11.3-1nodesource1_amd64.deb && \
+    apt remove -y cmdtest && \
     apt-get install -y yarn
 
 RUN cd /musicbrainz-server/ && eval $( perl -Mlocal::lib) && cpanm --installdeps --notest .