runtest: clone config before passing to write to avoid side effects

runtest: clone config before passing to write to avoid side effects

write changes some parts of the config, if this is a reference to the
config used for checking parsing it may lead to unexpected failures
due to those side effects..

For me it was "cidr" and "cidr6" getting deleted, and thus upcomming
tests for a compat change failing without any apparent reason.. :/

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>