test 'Serial Number arithmetic' edge cases involving RRSIG's {inception,expiration} fields

[japaric]

validating a RRSIG involves comparing the current time (seconds since UNIX_EPOCH) against the {inception,expiration} fields of the RRSIG (see section 5.3.1 of RFC4035). Section 3.1.5 of RFC4034 indicates that the {inception,expiration} are not plain unsigned-integers but rather 'Serial Numbers' whose comparison logic in specified in RFC1982.

we should test what the behavior of other DNS implementations is when the value of these 32-bit {inception,expiration} fields is 1 << 31 seconds beyond UNIX_EPOCH, 1 << 32 seconds beyond UNIX_EPOCH and even beyond that.