MNT Add tests for multi-WYSIWYG config regressions

[GuySartorelli]

Description

Tests for regressions for having multiple WYSIWYG configs.
Tests are for silverstripe/silverstripe-admin#1512 and #1120

Needs all three of the below PRs to be merged for the CI to go green:

• FIX Add tinymce link menuitems per editor instance silverstripe-admin#1653
• FIX Add tinymce link menuitems per editor instance silverstripe-asset-admin#1435
• FIX Add tinymce link menuitems per editor instance silverstripe-cms#2916

Manual testing steps

Load up an environment with the three linked PRs included, and run the new behat test.

Issues

• Link-dropdown breaks in TinyMCE when other EditorConfig is used on one of the Elements #1120

Pull request checklist

• The target branch is correct
  • See picking the right version
• All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• CI is green

[GuySartorelli]

This adds no benefit but does no harm in 4.x.
In 5.x it tells your IDE what $this->owner and $this->getOwner() are.

[emteknetnz]

Seems like this should be in admin, not elemental?

[GuySartorelli]

Relies on elemental blocks - has to be in elemental.

Update: I have tried to reproduce this without elemental and haven't been able to do so.

[emteknetnz]

Code is fine, CI is red

[GuySartorelli]

Rerunning CI - as per PR description it needed the others merged first before it could go green.