fix: Add missing stderr from assert_success output

[nieomylnieja]

Since we introduced run --separate-stderr flag some time ago in order to avoid assert_output checking both stdout and stderr (which can contain stuff like "retrying" notices) and failing due to that, it had a side effect on assert_success, which now only prints $output which is only stdout (stderr is separated into $stderr).

To fix that we need to give assert_success conjoined stdout and stderr.