tests: demonstrate how conflict markers end up $local *and* $output When a merge tool is configured to keep conflict markers, they are supposed to be written to `$local` if `$output` is not mentioned in the tool's `merge-tools.<tool>.args` config, and in `$output` if it is mentioned. However, I broke the latter case in D12190. Differential Revision: https://phab.mercurial-scm.org/D12378