Put back explicit setting of replication values within TAP tests.

Commit 151c0c5f neglected the possibility that a TEMP_CONFIG file
would explicitly set max_wal_senders=0; as indeed buildfarm member
thorntail does, so that it can test wal_level=minimal in other test
suites.  Hence, rather than assuming that max_wal_senders=10 will
prevail if we say nothing, set it explicitly.

Set max_replication_slots=10 explicitly too, just to be safe.

Back-patch to v10, like the previous patch.

Discussion: https://postgr.es/m/723911.1601417626@sss.pgh.pa.us

src/test/perl/PostgresNode.pm

@@ -469,12 +469,15 @@ sub init
 		{
 			print $conf "wal_level = replica\n";
 		}
-		print $conf "max_wal_size = 128MB\n";
+		print $conf "max_wal_senders = 10\n";
+		print $conf "max_replication_slots = 10\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters:
 		print $conf "shared_buffers = 1MB\n";
 		print $conf "max_connections = 10\n";
+		# limit disk space consumption, too:
+		print $conf "max_wal_size = 128MB\n";
 	}
 	else
 	{