Exercise the workspaces CLI in usage tests.

scripts/travis_script.sh

@@ -133,6 +133,11 @@ if [[ $GROUP == usage ]]; then
     jupyter labextension uninstall @jupyterlab/mock-extension --no-build
     jupyter labextension uninstall @jupyterlab/notebook-extension --no-build
     popd
+    jupyter lab workspaces export > workspace.json
+    jupyter lab workspaces import --name newspace workspace.json
+    jupyter lab workspaces export newspace > newspace.json
+    rm workspace.json newspace.json
+
 
     # Make sure we can call help on all the cli apps.
     jupyter lab -h