Fix database tests

Whooooot fixed in the 1567438-database-transaction branch!

Yay, thanks a lot!

I confirmed that the test failures are resolved.

Branch link: 1567438-database-transaction
Commit: 60ebceebf9607a63c

Diff for quick review:

diff --git a/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php b/core/lib/Drupal/Core/Database/Driver/mysq
index e56da55..2d3fffa 100644
--- a/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Connection.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Core\Database\Driver\mysql;
 
+use Drupal\Core\Database\DatabaseExceptionWrapper;
+
 use Drupal\Core\Database\Database;
 use Drupal\Core\Database\TransactionCommitFailedException;
 use Drupal\Core\Database\DatabaseException;
@@ -180,7 +182,7 @@ class Connection extends DatabaseConnection {
         try {
           $this->query('RELEASE SAVEPOINT ' . $name);
         }
-        catch (PDOException $e) {
+        catch (DatabaseExceptionWrapper $e) {
           // However, in MySQL (InnoDB), savepoints are automatically committed
           // when tables are altered or created (DDL transactions are not
           // supported). This can cause exceptions due to trying to release
@@ -188,7 +190,7 @@ class Connection extends DatabaseConnection {
           //
           // To avoid exceptions when no actual error has occurred, we silently
           // succeed for MySQL error code 1305 ("SAVEPOINT does not exist").
-          if ($e->errorInfo[1] == '1305') {
+          if ($e->getPrevious()->errorInfo[1] == '1305') {
             // If one SAVEPOINT was released automatically, then all were.
             // Therefore, clean the transaction stack.
             $this->transactionLayers = array();