Fix recovery test hang in 021_row_visibility.pl on windows.

The psql processes were not explicitly killed (but would eventually
exit due postgres shutting down). For some reason windows perl doesn't
like that, resulting in errors like
Warning: unable to close filehandle GEN20 properly: Bad file descriptor during global destruction.

The test was introduced in d6734a897e3, so no backpatching necessary.

src/test/recovery/t/021_row_visibility.pl

@@ -151,6 +151,10 @@ ok(send_query_and_wait(\%psql_standby,
 					   qr/will_commit.*\n\(1 row\)$/m),
    'finished prepared visible');
 
+# explicitly shut down psql instances - they cause hangs on windows
+$psql_primary{run}->kill_kill;
+$psql_standby{run}->kill_kill;
+
 $node_primary->stop;
 $node_standby->stop;