Debugging improvements

[kbond]

1. TestMailer::dump/dd(): ability to dump the sent emails to help debug problems.
2. TestMailer assertions should, as part of the failure message, output a summary of the emails sent if applicable. For instance, if TestMailer::assertNoEmailSent() fails, it should display a summary of emails that were sent.

[norkunas]

I'd suggest to add assertHtmlBodySame(expected) because now using assertHtmlContains on failure just says that it does not include expected html instead of giving any reasonable diff which is hard to debug. Then I need to switch to simple phpunit's assertSame, fix html and then revert 😑