tests: use consistent environment variables for setting charset

Character set in POSIX is set by the locale defined (in decreasing order
of precedence) by the LC_ALL, LC_CTYPE and LANG environment variables (I
believe CHARSET is only historic). 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 overriden, if set.  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
(where relevant) to skip the test if UTF-8 isn't in use.  Test 1035 was
also converted to UTF-8 for consistency, as the actual character set
used there is irrelevant to the test.