test: Send error messages to stderr, where they belong

When error messages are written to stdout, they will be considered as
output in case we want to fail from inside $( ) or backticks, and then
the error does not end up on the terminal.

test/util.sh

@@ -45,9 +45,9 @@ in_env() {
 
 fail_test() {
 	if in_env github; then
-		echo "::error::$@"
+		echo "::error::$@" >&2
 	else
-		echo -e "FAIL: $@"
+		echo -e "FAIL: $@" >&2
 	fi
 	exit 1
 }