tests: Only run xdg-document-portal in foreground when uninstalled

During build-time testing, we need to take special steps to run the
just-built version of the portal. However, during "as-installed"
testing we want to assert that it was installed correctly, and in
particular that D-Bus .service file was installed correctly to make it
activatable.

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