When libidn2 2.3.0+ is used, the "idna" system test fails as dig does
not fail as expected any more due to libidn2 2.3.0 using Unicode 11.0.0
IDNA mapping tables [1] instead of Unicode 6.3.0 ones which were used
until libidn2 2.2.0. Specifically, the SOCKS character (🧦) used in the
invalid input U-label check in the "idna" system test is a valid IDNA
character as per the IDNA mapping tables for Unicode 11.0.0. Fix the
relevant checks by using a different character (🟥) which is an invalid
IDNA character for all versions of Unicode released to date.
[1] 5eaafda64e
367a576f73