Don't auto clean @theia/test package

Signed-off-by: Timothy Johnson <timothy.johnson@broadcom.com>

ze/theia-slim/Dockerfile

@@ -23,6 +23,7 @@ RUN yarn global add node-gyp && \
     echo *.ts >> .yarnclean && \
     echo *.ts.map >> .yarnclean && \
     echo *.spec.* >> .yarnclean && \
+    echo "!@theia/test" >> .yarnclean && \
     yarn autoclean --force && \
     yarn cache clean
 # Uncomment the following lines to install Zowe Explorer in the container