test_dhcp: Start a dhcp helper functions UT

test_dhcp: Start a dhcp helper functions UT

Use extra define to allow testing code only
used on Windows but not actually dependent
on Windows.

Change-Id: I08e50030b1b692d351509f541e5c0b03b5170615
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1267
Message-Id: <20251013154758.21695-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59246199/
Signed-off-by: Gert Doering <gert@greenie.muc.de>