verify that postBuild is run in tests/venv/start/postBuild

tests/venv/start/postBuild/postBuild

@@ -2,3 +2,5 @@
 
 # this value should not be visible in `verify`
 export TEST_START_VAR="var is set by postBuild"
+
+echo 'Done!' > $HOME/postbuild.txt

tests/venv/start/postBuild/verify

@@ -1,7 +1,6 @@
 #!/bin/bash
 set -euo pipefail
-jupyter nbextension list | grep 'jupyter-leaflet' | grep enabled
-
+grep 'Done!' $HOME/postbuild.txt
 # set value of TEST_START_VAR to empty string when it is not defined
 if [ "${TEST_START_VAR:-}" != "var is set" ]
 then