Internal: Allow debugging mail spool for failed tests

[Kingdutch]

Problem / Solution

It can be difficult to debug failed tests for e-mails because we are not able to look at the files that are in the mail-spool. This commit changes the Behat failure scenario to also copy any e-mail messages from the mail-spool on a test failure.

Issue tracker

Internal change no ticket

How to test

• Fail a behat email test in the CI and see its output in the attachment

Definition of done

Before merge

• Code/peer review is completed
• All commit messages are clear and clean. If applicable a rebase was performed
• All automated tests are green
• Functional/manual tests of the acceptance criteria are approved
• All acceptance criteria were met
• New features or changes to existing features are covered by tests, either unit (preferably) or behat
• Update path is tested. New hook_updates should respect update order, right naming convention and consider hook_post_update code
• Module can be safely uninstalled. Update/implement hook_uninstall and make sure that removed configuration or dependencies are removed/uninstalled
• This pull request has all required labels (team/type/priority)
• This pull request has a milestone
• This pull request has an assignee (if applicable)
• Any front end changes are tested on all major browsers
• New UI elements, or changes on UI elements are approved by the design team
• New features, or feature changes are approved by the product owner

After merge

• Code is tested on all branches that it has been cherry-picked
• Update hook number might need adjustment, make sure they have the correct order
• The Drupal.org ticket(s) are updated according to this pull request status

[mergeable]

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[robertragas]

Approved, tested it in the replace swiftmailer story where the tests where failing and wasn't sure why. This changed helped me to see the emails that were sent and pinpoint the issue itself.