The test expected `xn--ah-` to be treated as a syntax error (punycode
requires letters after the last hyphen) but libidn2 on buster
converted the label to `ah` instead. To avoid this bug, change the
invalid label to `xn--0000h` which translates to an out-of-range
unicode codepoint (beyond the maximum value) which is corectly
trated as invalid in older libidn2.
5c1aed2881