Further tweaks to stuck_on_old_timeline recovery test

Further tweaks to stuck_on_old_timeline recovery test

Translate path slashes on target directory path. This was confusing old
branches, but is applied to all branches for the sake of uniformity.
Perl is perfectly able to understand paths with forward slashes.

Along the way, restore the previous archive_wait query, for the sake of
uniformity with other tests, per gripe from Tom Lane.