The nm_dhcp_dhclient_save_duid() function will save a newly generated
DUID to a previously existing lease file. The function will only save
the DUID if not present in the lease file: in this case, should preserve
the other contents of the lease file.
A dhclient lease file for IPv6 generated by NetworkManager will always
add the DUID as a first item: so in practice finding a lease file
without DUID will never happen.
This has hidden a bug in the function: the loop that is meant to append
the non-duid lines in the lease file would strip all the newlines,
mangling the lease file.
Fix the function allowing to keep the original lines and add a test to
check this functionality is kept well functioning.

FIXME: the new test and the other duid ones already there  store the file
in the current working-directory. Tests should not do that.