Let the tests replace the database connection class.

open_db.lib.php

@@ -1,8 +1,12 @@
 <?php
 require_once(__DIR__ . '/config.lib.php');
 global $db;
+/*
+ * The class of the connection that open_db() makes. It is always mysqli
+ * on the site; the tests put a fake class in the global $db_class.
+ */
 function open_db($state) {
-  global $db;
+  global $db, $db_class;
   if ($state == 'open') {
     if (!$db) {
       $hostname  = config('DB_HOST', 'localhost');
@@ -11,7 +15,8 @@ function open_db($state) {
       $db_name   = config('DB_NAME');
       // PHP 8.1+ throws exceptions by default; keep the old return-value checks working
       mysqli_report(MYSQLI_REPORT_OFF);
-      $db = new mysqli($hostname, $view_user, $view_pass, $db_name);
+      $class = $db_class ?? 'mysqli';
+      $db = new $class($hostname, $view_user, $view_pass, $db_name);
       if ($db->connect_error) {
         $detail = 'connect error (' . $db->connect_errno . ') ' . $db->connect_error;
         $db = NULL;