Use a short socket directory path in pg_upgrade testing.

Several buildfarm members are failing the pg_upgrade test in
REL_15_STABLE, though the identical test is fine in HEAD.
On thorntail it's possible to see that the problem is an
overlength socket path name, and I bet the same is true
on the others.

The normally-started postmasters used in the test are already
set up with short socket directory paths, but we neglected to
tell pg_upgrade itself to do likewise when starting child
postmasters, and indeed it seems to be explicitly selecting
the test directory instead.

Back-patch to v15 where the current test script was introduced.
(The previous script might have the same issue, because I don't
see any use of -s/--socketdir in it either; but we've had no
complaints, so leave it alone for now.)

Discussion: https://postgr.es/m/1410025.1656890531@sss.pgh.pa.us

Author: tglsfdc

src/bin/pg_upgrade/t/002_pg_upgrade.pl

@@ -226,6 +226,7 @@ sub generate_db
 		'-D',         $newnode->data_dir,
 		'-b',         $oldbindir . '/does/not/exist/',
 		'-B',         $newbindir,
+		'-s',         $newnode->host,
 		'-p',         $oldnode->port,
 		'-P',         $newnode->port,
 		'--check'
@@ -240,8 +241,9 @@ sub generate_db
 	[
 		'pg_upgrade', '--no-sync',        '-d', $oldnode->data_dir,
 		'-D',         $newnode->data_dir, '-b', $oldbindir,
-		'-B',         $newbindir,         '-p', $oldnode->port,
-		'-P',         $newnode->port,     '--check'
+		'-B',         $newbindir,         '-s', $newnode->host,
+		'-p',         $oldnode->port,     '-P', $newnode->port,
+		'--check'
 	],
 	'run of pg_upgrade --check for new instance');
 ok(!-d $newnode->data_dir . "/pg_upgrade_output.d",
@@ -252,8 +254,8 @@ sub generate_db
 	[
 		'pg_upgrade', '--no-sync',        '-d', $oldnode->data_dir,
 		'-D',         $newnode->data_dir, '-b', $oldbindir,
-		'-B',         $newbindir,         '-p', $oldnode->port,
-		'-P',         $newnode->port
+		'-B',         $newbindir,         '-s', $newnode->host,
+		'-p',         $oldnode->port,     '-P', $newnode->port
 	],
 	'run of pg_upgrade for new instance');
 ok( !-d $newnode->data_dir . "/pg_upgrade_output.d",