Take redirects off the Host header.

Same-site redirects are now relative paths, and a saved page on another site is ignored. The HTTP to HTTPS redirect takes its host from the new SITE_URL setting, and without it the pages refuse plain HTTP.

tests/php/Support/PageTestCase.php

@@ -15,6 +15,11 @@ abstract class PageTestCase extends \PHPUnit\Framework\TestCase {
   protected static string $base;
   private static $server = NULL;
 
+  // Environment variables for the server, on top of the usual ones
+  protected static function serverEnv(): array {
+    return array();
+  }
+
   // The cookies that the site set, name => value
   protected array $cookies = array();
 
@@ -43,7 +48,7 @@ abstract class PageTestCase extends \PHPUnit\Framework\TestCase {
       'XXX_TEST_LOG'    => self::$state . '/php.log',
       'MAIL_LOG'        => self::$state . '/mail.jsonl',
       'UPLOAD_DIR'      => self::$state . '/tmp/uploads',
-    );
+    ) + static::serverEnv();
     $log = array('file', self::$state . '/server.log', 'a');
     self::$server = proc_open($command, array(0 => array('file', '/dev/null', 'r'), 1 => $log, 2 => $log), $pipes, XXX_ROOT, $env);