Split saturnin-demo to a double-package distribution, where one package provides core saturnin-demo with its subcommands and the other provides a Shellfu/Bash module saturnin_demo_greet.sh used by the first one.
Tests are supposed to set it on their own and not rely on environment. Hence it's not even interesting to have in TF_HEADER.
This requires newer Saturnin version in order to avoid bug where SATURNIN_CONF_PATH environment variable would be ignored.