Adjust new test case to set wal_keep_segments.

Per buildfarm member conchuela and Kyotaro Horiguchi, it's possible
for the WAL segment that the cascading standby needs to be removed
too quickly. Hopefully this will prevent that.

Kyotaro Horiguchi

Discussion: http://postgr.es/m/20210610.101240.1270925505780628275.horikyota.ntt@gmail.com

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

@@ -27,6 +27,7 @@ $perlbin =~ s{\\}{\\\\}g if ($TestLib::windows_os);
 my $archivedir_primary = $node_primary->archive_dir;
 $node_primary->append_conf('postgresql.conf', qq(
 archive_command = '$perlbin "$FindBin::RealBin/cp_history_files" "%p" "$archivedir_primary/%f"'
+wal_keep_segments=8
 ));
 $node_primary->start;