Test system bus config files on Unix only

Test system bus config files on Unix only

Previously, we didn't consistently test parsing of every file in
valid-config-files-system/ everywhere that we tested valid-config-files/.
We now test it on Unix.

The system bus is not supported on Windows, so we do not test
valid-config-files-system/ there.

valid-config-files/many-rules.conf contains <user> and <group> rules
which are not applicable to Windows. Copy the original many-rules.conf
to valid-config-files-system/ so that it will be tested on Unix, and
remove the non-portable rules from valid-config-files/many-rules.conf.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=92721
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
[rh:base patch came from Simon]