tests: Don't reset XDG_RUNTIME_DIR locally

If we do, it interferes with xdg-dbus-proxy, causing test failure under
some circumstances: the test passes on a development system, but fails
when run on a qemu virtual machine in Debian's autopkgtest framework.

Fixes: 6e5b02e2 "run: Don't let XDG_RUNTIME_DIR from user override the value we set"
Signed-off-by: Simon McVittie <smcv@collabora.com>