Cover forum post inline/edit formats, editors with automated tests

This is about to cover MDL-65641 (together surely with MDL-65635) with some good behat tests.

There are a number of formats and editors, and 2 actions: edit existing post and advanced edition (from inline-reply-to-post) that should be covered, ensuring, basically, that:

1) format always wins when we are handling existing content.
2) preferred editor format is the one used for new content.
3) editor is only shown when it supports the format being used.

It doesn't seem that the tests are hard at all, just playing with a preference and looking for results when editing a existing post (36 and 37) and when going to advance for a new post (37 only).

Better we have them covered to ensure no change in behavior is introduced there without noticing.

Ciao