Behat file uploads patch for theme_boost on Moodle >= 3.2

[LukeCarrier]

@sliute and I today encountered issues with failing file uploads running acceptance tests with Moodle 3.3 running theme_boost. We crudely ported the patch from behat_repository_upload to the theme_boost equivalent, which works.

Suggested steps:

• Update the patch linked in the documentation
• Attempt to get it upstreamed and backported to all support branches

[LukeCarrier]

Partially addressed in f279025 -- we now need to pursue getting something upstreamed after some internal testing.