testlibrary: Correct a wrong string in a debug message
testlibrary: Correct a wrong string in a debug message

This always printed "setting HOME=(null)" because we didn't set
datadir yet, but it should have been looking at homedir anyway.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #1760
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak