Add rudimentary check for ehrQL in dev container

As the comment states, we cannot check this is installed properly yet.

tests/dev_container.sh

@@ -6,3 +6,9 @@ opensafely run run_all
 
 # Check the RStudio server is running.
 curl -L 'http://localhost:8787' | grep 'RStudio'
+
+# This is a rudimentary placeholder test of the ehrQL installation.
+# We cannot easily run ehrQL because it requires Python 3.11 to be imported,
+# but the OpenSAFELY Python Docker image, and the dev container, uses Python 3.10.
+# In future, we could actually try and import ehrQL through Python.
+ls .devcontainer/ehrql-main/