Fix return type for TestEmailTransport::send().
Fix return type for TestEmailTransport::send().

Closes #13510.

ADmad/cakephp,mvdriel/cakephp,mvdriel/cakephp,CakeDC/cakephp,cakephp/cakephp,ADmad/cakephp,ndm2/cakephp,ndm2/cakephp,CakeDC/cakephp,ndm2/cakephp,CakeDC/cakephp,cakephp/cakephp,ndm2/cakephp,mvdriel/cakephp,ADmad/cakephp,mvdriel/cakephp,cakephp/cakephp,cakephp/cakephp,ADmad/cakephp,CakeDC/cakephp