nbcelltests is designed for writing tests for linearly executed notebooks. Its primary use is for unit testing reports. To use in JupyterLab, you will also need the lab and server extensions. Typically, these are automatically installed alongside nbcelltests, so you should not need to do anything special to use them. The lab extension will require a rebuild of JupyterLab, which you'll be prompted to do on starting JupyterLab the first time after installing celltests (or you can do manually with jupyter lab build). Note that you must have node.js installed (as for any lab extension).