REGTESTS: http-rules: cover the rfc7239_nn and rfc7239_np converters

REGTESTS: http-rules: cover the rfc7239_nn and rfc7239_np converters

The test already covers rfc7239_field, rfc7239_is_valid, rfc7239_n2nn
and rfc7239_n2np, and the two converters which build a node name and a
node port were the only ones of the family left out. They are added to
the backend which already extracts the node of a "for" field, with one
IPv4, one IPv6, one string and one integer input, which covers both
branches of each: an address or a number is emitted as it is, an IPv6
is bracketed and a string becomes an obfuscated identifier prefixed
with an underscore.