The test binary has two modes: in the default argument-less mode, it
just checks that "root" can be resolved. When invoked manually, a root
prefix and user/group names can be specified.
Use -Dstandalone-binaries=yes to enable building and installing this standalone
version of the binary without a dependency on the systemd-shared solib.
Also move the list of sources for systemd-tmpfiles to its own meson.build file.