Create unit test for change the phone number prefix.

https://bugzilla.gnome.org/show_bug.cgi?id=731175
Create a new function set_string_afd_equal.

This is necessary to avoid problems with phone numbers that has a special values_equal function to try match phone numbers in different formats.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=731175