meson: use conf configuration_data object to generate test-sysusers.sh

meson: use conf configuration_data object to generate test-sysusers.sh

I wanted to use jinja2 templating here too, but it's hard to get right:
custom_target() strips the executable bit by default (unlike configure_file
apparently). custom_target() has install_mode setting, but it was only added
in meson-0.47, so it can't be used while we support 0.46. And without the
executable bit the test is not invoked properly. For example, "root-unittests"
in the debian package calls test-* after installation, so the executable bit
there is necessary. It would be possible to adjust the file mode after the
fact, but it would make things more complicated.

So let's use the native meson substitutions here. We don't need anything more
fancy.