[9.18] fix: usr: fix generation of 6to4-self name expansion from IPv4 address

The period between the most significant nibble of the encoded IPv4 address and the 2.0.0.2.IP6.ARPA suffix was missing resulting in the wrong name being checked. Add system test for 6to4-self implementation.

Closes #4766

Backport of MR !9099

Merge branch 'backport-4766-add-system-test-for-6to4-self-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9218