Set DBSetupTest timezone to UTC (#12881)

Not sure this is correct, but this test fails 100% on my local system since my timezone is not UTC

tests/DBSetupTest.php

@@ -123,6 +123,8 @@ class DBSetupTest extends DBTestCase
     public function testValidateSchema()
     {
         if (is_file('misc/db_schema.yaml')) {
+            DB::connection($this->connection)->statement('SET time_zone = "+00:00";');
+
             $master_schema = \Symfony\Component\Yaml\Yaml::parse(
                 file_get_contents('misc/db_schema.yaml')
             );