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.

src/test/recovery/t/025_stuck_on_old_timeline.pl

@@ -31,6 +31,7 @@ if ($^O eq 'msys')
 }
 $perlbin =~ s!\\!/!g if $TestLib::windows_os;
 my $archivedir_primary = $node_primary->archive_dir;
+$archivedir_primary =~ s!\\!/!g if $TestLib::windows_os;
 $node_primary->append_conf('postgresql.conf', qq(
 archive_command = '"$perlbin" "$FindBin::RealBin/cp_history_files" "%p" "$archivedir_primary/%f"'
 wal_keep_size=128MB
@@ -81,8 +82,7 @@ $node_standby->safe_psql('postgres', 'SELECT pg_switch_wal()');
 # WAL segment, this is enough to guarantee that the history file was
 # archived.
 my $archive_wait_query =
-  "SELECT coalesce('$walfile_to_be_archived' <= last_archived_wal, false) " .
+  "SELECT '$walfile_to_be_archived' <= last_archived_wal FROM pg_stat_archiver";
-  "FROM pg_stat_archiver";
 $node_standby->poll_query_until('postgres', $archive_wait_query)
   or die "Timed out while waiting for WAL segment to be archived";
 my $last_archived_wal_file = $walfile_to_be_archived;