(maint) Fix potential merge conflict

Prior to this commit, there was a potential merge conflict between
the two definitions of `shared_examples_for "ifconfig output"` in
facter/spec/unit/ipaddress_spec.rb and facter/spec/unit/util/ip_spec.rb.
This commit fixes that by changing the definition in
facter/spec/unit/util/ip_spec.rb to `shared_examples_for "ifconfig
output test"`