Some tests assumed "iconv" is available without honoring ICONV
prerequisite, which has been corrected.
* ps/tests-wo-iconv-fixes:
t6006: don't use iconv(1) without ICONV prereq
t5550: add ICONV prereq to tests that use "$HTTPD_URL/error"
t4205: improve handling of ICONV prerequisite
t40xx: don't use iconv(1) without ICONV prereq
t: don't set ICONV prereq when iconv(1) is missing