Internal: Ensure the mail-spool directory exists for Behat tests

[Kingdutch]

Problem / Solution

We previously created this in the GitHub workflow, but this makes new developer project set-up more difficult (because then when running an email test the folder doesn't exist).

By adding it to the repository we ensure it's present for developers and we can remove some steps from the CI.

We rename it from swiftmailer-spool to mail-spool because we know in the future we want to move away from swiftmailer.

We ignore all messages in the directory to make sure we don't accidentally commit debug output.

Issue tracker

Internal no issue

How to test

• Behat CI should still pass

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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