Update postBuild+start test case

This test was missing a start script, added it. Updated the README to
explain what the test case was doing.

tests/venv/start/postBuild/README.rst

@@ -1,8 +1,5 @@
-System - Post-build scripts
----------------------------
-
-It is possible to run scripts after you've built the environment specified in
-your other files. This could be used to, for example, download data or run
-some configuration scripts.
-
-In this example, we download and install a Jupyter Notebook extension.
+postBuild and start
+-------------------
+
+This test checks that we can use a postBuild and start script
+at the same time.

tests/venv/start/postBuild/postBuild

@@ -1,2 +1,6 @@
 #!/bin/bash
+
+# this value should not be visible in `verify`
+export TEST_START_VAR="var is set by postBuild"
+
 jupyter nbextension enable --py --sys-prefix ipyleaflet

tests/venv/start/postBuild/start

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+export TEST_START_VAR="var is set"
+exec "$@"

tests/venv/start/postBuild/verify

@@ -2,7 +2,8 @@
 set -euo pipefail
 jupyter nbextension list | grep 'jupyter-leaflet' | grep enabled
 
-if [ "$TEST_START_VAR" != "var is set" ]
+# set value of TEST_START_VAR to empty string when it is not defined
+if [ "${TEST_START_VAR:-}" != "var is set" ]
 then
   echo "TEST_START_VAR is not set"
   exit 1