following @igorw approach

tests/Silex/Tests/Provider/SwiftmailerServiceProviderTest.php

@@ -49,33 +49,28 @@ class SwiftmailerServiceProviderTest extends \PHPUnit_Framework_TestCase
 
         $test = $this;
 
-        /**
-         * This gets executed before SwiftmailerServiceProvider $app->finish thanks to higher priority
-         * flushQueue should return 1 if spool has not been flushed
-         */
-        $app->finish(function() use ($app, $test) {
-            $test->assertEquals(1, $app['swiftmailer.spooltransport']->getSpool()->flushQueue($app['swiftmailer.transport']));
-            /**
-             * We add a new message that should be flushed with $app->finish()
-             */
+        $app->get('/', function() use ($app) {
             $app['mailer']->send(\Swift_Message::newInstance());
-        }, 1);
+        });
 
         /**
-         * This gets executed after SwiftmailerServiceProvider $app->finish thanks to higher priority
-         * flushQueue should return 0 even having added a message in method above
+         * Checks spool is empty before process
          */
-        $app->finish(function() use ($app, $test) {
-            $test->assertEquals(0, $app['swiftmailer.spooltransport']->getSpool()->flushQueue($app['swiftmailer.transport']));
-        }, -1);
-
-        $app->get('/', function() use ($app) {
-            $app['mailer']->send(\Swift_Message::newInstance());
-            return new SendMailsResponse('should send e-mails');
-        });
+        $test->assertEquals(0, $app['swiftmailer.spooltransport']->getSpool()->flushQueue($app['swiftmailer.transport']));
 
         $request = Request::create('/');
-        $app->run($request);
+        $app->handle($request);
+        /**
+         * Checks spool has the message that is sent in controller and regenerates it
+         */
+        $test->assertEquals(1, $app['swiftmailer.spooltransport']->getSpool()->flushQueue($app['swiftmailer.transport']));
+        $app['mailer']->send(\Swift_Message::newInstance());
+
+        /**
+         * Terminates app and checks that spool is empty
+         */
+        $app->terminate($request, new SendMailsResponse('should send e-mails'));
+        $test->assertEquals(0, $app['swiftmailer.spooltransport']->getSpool()->flushQueue($app['swiftmailer.transport']));
     }
 }