Hide expensive pg_upgrade test behind PG_TEST_EXTRA

This new test is very expensive.  Make it opt-in.

Discussion: https://postgr.es/m/202508051433.ebznuqrxt4b2@alvherre.pgsql

doc/src/sgml/regress.sgml

@@ -284,6 +284,19 @@ make check-world PG_TEST_EXTRA='kerberos ldap ssl load_balance libpq_encryption'
      </listitem>
     </varlistentry>
 
+    <varlistentry>
+     <term><literal>regress_dump_restore</literal></term>
+     <listitem>
+      <para>
+       Runs an additional test suite in
+       <filename>src/bin/pg_upgrade/t/002_pg_upgrade.pl</filename> which
+       cycles the regression database through <command>pg_dump</command>/
+       <command>pg_restore</command>.  Not enabled by default because it
+       is resource intensive.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><literal>sepgsql</literal></term>
      <listitem>

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

@@ -375,6 +375,9 @@ SKIP:
 {
 	my $dstnode = PostgreSQL::Test::Cluster->new('dst_node');
 
+	skip "regress_dump_restore not enabled in PG_TEST_EXTRA"
+	  if (!$ENV{PG_TEST_EXTRA}
+		|| $ENV{PG_TEST_EXTRA} !~ /\bregress_dump_restore\b/);
 	skip "different Postgres versions"
 	  if ($oldnode->pg_version != $dstnode->pg_version);
 	skip "source node not using default install"