tests(mingw): if `iconv` is unavailable, use `test-helper --iconv`

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/test-lib.sh

@@ -1712,6 +1712,12 @@ case $uname_s in
 	test_set_prereq GREP_STRIPS_CR
 	test_set_prereq WINDOWS
 	GIT_TEST_CMP="GIT_DIR=/dev/null git diff --no-index --ignore-cr-at-eol --"
+	if ! type iconv >/dev/null 2>&1
+	then
+		iconv () {
+			test-tool iconv "$@"
+		}
+	fi
 	;;
 *CYGWIN*)
 	test_set_prereq POSIXPERM