Speed up TAP tests of pg_receivewal

This commit improves the speed of those tests by 25~30%, using some
simple ideas to reduce the amount of data written by pg_receivewal:
- Use a segment size of 1MB.  While reducing the amount of data zeroed
by pg_receivewal for the new segments, this improves the code coverage
with a non-default segment size.
- In the last test involving a slot's restart_lsn, generate a checkpoint
to advance the redo LSN and the WAL retained by the slot created,
reducing the number of segments that need to be archived.  This counts
for most of the gain.
- Minimize the amount of data inserted into the dummy table.

Reviewed-by: Ronan Dunklau
Discussion: https://postgr.es/m/YXqYKAdVEqmyTltK@paquier.xyz

src/bin/pg_basebackup/t/020_pg_receivewal.pl

@@ -15,7 +15,7 @@ program_options_handling_ok('pg_receivewal');
 umask(0077);
 
 my $primary = PostgreSQL::Test::Cluster->new('primary');
-$primary->init(allows_streaming => 1);
+$primary->init(allows_streaming => 1, extra => ['--wal-segsize=1']);
 $primary->start;
 
 my $stream_dir = $primary->basedir . '/archive_wal';
@@ -50,13 +50,12 @@ is($primary->slot($slot_name)->{'slot_type'},
 # Generate some WAL.  Use --synchronous at the same time to add more
 # code coverage.  Switch to the next segment first so that subsequent
 # restarts of pg_receivewal will see this segment as full..
-$primary->psql('postgres', 'CREATE TABLE test_table(x integer);');
+$primary->psql('postgres', 'CREATE TABLE test_table(x integer PRIMARY KEY);');
 $primary->psql('postgres', 'SELECT pg_switch_wal();');
 my $nextlsn =
   $primary->safe_psql('postgres', 'SELECT pg_current_wal_insert_lsn();');
 chomp($nextlsn);
-$primary->psql('postgres',
-	'INSERT INTO test_table VALUES (generate_series(1,100));');
+$primary->psql('postgres', 'INSERT INTO test_table VALUES (1);');
 
 # Stream up to the given position.  This is necessary to have a fixed
 # started point for the next commands done in this test, with or without
@@ -85,8 +84,7 @@ SKIP:
 	$nextlsn =
 	  $primary->safe_psql('postgres', 'SELECT pg_current_wal_insert_lsn();');
 	chomp($nextlsn);
-	$primary->psql('postgres',
-		'INSERT INTO test_table VALUES (generate_series(100,200));');
+	$primary->psql('postgres', 'INSERT INTO test_table VALUES (2);');
 
 	# Note the trailing whitespace after the value of --compress, that is
 	# a valid value.
@@ -136,8 +134,7 @@ $primary->psql('postgres', 'SELECT pg_switch_wal();');
 $nextlsn =
   $primary->safe_psql('postgres', 'SELECT pg_current_wal_insert_lsn();');
 chomp($nextlsn);
-$primary->psql('postgres',
-	'INSERT INTO test_table VALUES (generate_series(200,300));');
+$primary->psql('postgres', 'INSERT INTO test_table VALUES (3);');
 $primary->command_ok(
 	[
 		'pg_receivewal', '-D',     $stream_dir, '--verbose',
@@ -167,7 +164,9 @@ mkdir($slot_dir);
 $slot_name = 'archive_slot';
 
 # Setup the slot, reserving WAL at creation (corresponding to the
-# last redo LSN here, actually).
+# last redo LSN here, actually, so use a checkpoint to reduce the
+# number of segments archived).
+$primary->psql('postgres', 'checkpoint;');
 $primary->psql('postgres',
 	"SELECT pg_create_physical_replication_slot('$slot_name', true);");
 
@@ -181,13 +180,16 @@ my $walfile_streamed = $primary->safe_psql(
 
 # Switch to a new segment, to make sure that the segment retained by the
 # slot is still streamed.  This may not be necessary, but play it safe.
-$primary->psql('postgres',
-	'INSERT INTO test_table VALUES (generate_series(1,100));');
+$primary->psql('postgres', 'INSERT INTO test_table VALUES (4);');
 $primary->psql('postgres', 'SELECT pg_switch_wal();');
 $nextlsn =
   $primary->safe_psql('postgres', 'SELECT pg_current_wal_insert_lsn();');
 chomp($nextlsn);
 
+# Add a bit more data to accelerate the end of the next pg_receivewal
+# commands.
+$primary->psql('postgres', 'INSERT INTO test_table VALUES (5);');
+
 # Check case where the slot does not exist.
 $primary->command_fails_like(
 	[