test: cover nested transactions

_I managed to break this behaviour in other PR so this should be covered._

It tests basic walkthrough where we nest transactions, propagate the result through the transaction stack and also that it is committed and not rolled back.

tests/Functional/TransactionTest.php

@@ -7,7 +7,9 @@
 use Doctrine\DBAL\Connection;
 use Doctrine\DBAL\Exception\ConnectionLost;
 use Doctrine\DBAL\Platforms\AbstractMySQLPlatform;
+use Doctrine\DBAL\Schema\Table;
 use Doctrine\DBAL\Tests\FunctionalTestCase;
+use Doctrine\DBAL\Types\Types;
 
 use function func_get_args;
 use function restore_error_handler;
@@ -18,15 +20,6 @@
 
 class TransactionTest extends FunctionalTestCase
 {
-    protected function setUp(): void
-    {
-        if ($this->connection->getDatabasePlatform() instanceof AbstractMySQLPlatform) {
-            return;
-        }
-
-        self::markTestSkipped('Restricted to MySQL.');
-    }
-
     public function testCommitFailure(): void
     {
         $this->expectConnectionLoss(static function (Connection $connection): void {
@@ -43,6 +36,10 @@ public function testRollbackFailure(): void
 
     private function expectConnectionLoss(callable $scenario): void
     {
+        if (! $this->connection->getDatabasePlatform() instanceof AbstractMySQLPlatform) {
+            self::markTestSkipped('Restricted to MySQL.');
+        }
+
         $this->connection->executeStatement('SET SESSION wait_timeout=1');
         $this->connection->beginTransaction();
 
@@ -67,4 +64,30 @@ private function expectConnectionLoss(callable $scenario): void
             restore_error_handler();
         }
     }
+
+    public function testNestedTransactionWalkthrough(): void
+    {
+        $table = new Table('storage');
+        $table->addColumn('test_int', Types::INTEGER);
+        $table->setPrimaryKey(['test_int']);
+
+        $this->dropAndCreateTable($table);
+
+        $query = 'SELECT count(test_int) FROM storage';
+
+        self::assertSame('0', (string) $this->connection->fetchOne($query));
+
+        $result = $this->connection->transactional(
+            static fn (Connection $connection) => $connection->transactional(
+                static function (Connection $connection) use ($query) {
+                    $connection->insert('storage', ['test_int' => 1]);
+
+                    return $connection->fetchOne($query);
+                },
+            ),
+        );
+
+        self::assertSame('1', (string) $result);
+        self::assertSame('1', (string) $this->connection->fetchOne($query));
+    }
 }