Write Behat tests for Tiny editor

Write Behat tests to tests to the new Tiny editor been re-introduced in MDL-75071

Discussing with Andrew, there some suggestions of what should be done as part of this issue:

• Standard editor features is mostly covered already by the text formatting Andrew added.
• There's a commit to add support for the Image dialogue.
• Add support for the Video dialogue too.
• RecordRTC can't be automated tested sadly
• Ideally H5P plugin can be tested
• Investigate what can be tested for the accessibility checker
• Investigate what can be tested for the equation editor

This set of commits adds some behat steps and testing for Tiny.

There are some additional tests not yet included to test standard formatting functinoality of TinyMCE itself, but I'm currently experiencing some random failures with these so I'm going to defer them to a followup issue. Behat tests are allowed to land and be backported to stables regardless.