Otherwise tests can't pass, especially not on clean machine.

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.