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

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

t/test-lib.sh

@@ -1545,6 +1545,12 @@ case $uname_s in
 	test_set_prereq GREP_STRIPS_CR
 	test_set_prereq WINDOWS
 	GIT_TEST_CMP="test-tool cmp"
+	if ! type iconv >/dev/null 2>&1
+	then
+		iconv () {
+			test-tool iconv "$@"
+		}
+	fi
 	;;
 *CYGWIN*)
 	test_set_prereq POSIXPERM