testlibrary: Don't assume native architecture is x86_64
testlibrary: Don't assume native architecture is x86_64

Signed-off-by: Simon McVittie <smcv@debian.org>

Closes: #2353
Approved by: matthiasclasen

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