MDL-43757 messaging: Behat feature to test participants messaging

The new test checks that messages can be sent from
the participants block and they can be read when
accesing user's recent conversations.
MDL-43757 behat: Allowing the form field guesser to detect TinyMCE

It includes 2 more minor fixes:
- Escapes values sent to TinyMCE as it was breaking
  when values contained double quotes.
- Adding seconds to the screenshots directory to
  avoid using the same folder for multiple runs.