tests: use consistent environment variables for setting charset

The character set in POSIX is set by the locale defined by (in
decreasing order of precedence) the LC_ALL, LC_CTYPE and LANG
environment variables (CHARSET was used by libidn but not libidn2).
LC_ALL is cleared to ensure that LC_CTYPE takes effect, but LC_ALL is
not used to set the locale to ensure that other parts of the locale
aren't overridden.  Since there doesn't seem to be a cross-platform way
of specifying a UTF-8 locale, and not all systems may support UTF-8, a
<precheck> is used to skip the test if UTF-8 can't be verified to be
available.  Test 1035 was also converted to UTF-8 for consistency, as
the actual character set used there is irrelevant to the test.

This patch uses a different UTF-8 locale than the last attempt, namely
en_US.UTF-8. This one has been verified on 7 different Linux and BSD
distributions and is more complete and usable than the locale UTF-8 (on
at least some systems).