tests: Install test.portal for installed-tests
tests: Install test.portal for installed-tests

Otherwise, when running installed-tests, xdg-desktop-portal will
find a real installed backend such as xdg-desktop-portals-gtk (if any),
rather than using the intended test backend.

Fixes: https://github.com/flatpak/xdg-desktop-portal/issues/431
Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@debian.org>

matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal