fix: PHP warnings triggered by the test suite

The test should fail when a warning PHP error occurred.

Depends on

To make test passes, I apply the following dependencies:

Then execute:

test/bin/test --dependency-preferences 'lowest highest'

Set consistent error_reporting to catch from PHP warnings and higher errors.
[validator] Fix two kind of warning
[mailer] fix the support for swiftmailler 6
- An alternative with tests for swiftmailer fix #279
- The default sfMailer transport for Switf v6 is Swift_SmtpTransport instead of Swift_MailTransport.
- As Swift_MailTransport as been removed.
- Use composer version of swiftmailler by default on test.
- Add test without composer
- Remove duplicated code for loading from submodule
Update core autoloader, and apply 4 space indents.
- data/bin/symfony symfony:test --trace --update-autoloader

I try to put a PHP error_reporting directive, but the execution environment looks like different than docker.