gnutls: ignore invalid certificate dates with VERIFYPEER disabled

This makes the behaviour consistent with what happens if a date can
be extracted from the certificate but is expired.
tests: document more test identifiers and variables
test320/1/2/4: fix the port number substitution variables

These tests have been broken since commit 1958fe57 in Oct. 2011
tests: adjust for capitalization differences in newer gnutls-serv