Move block content creation methods to a trait

Converting FieldTypeTest was completed in 3414259. The remaining part here is to create a trait for the creation of blocks.

I am changing the title for that new goal and removing the parent.

Started this from scratch as much of the patch no longer applied. I've updated other tests that call something similar to createBlockContent with the trait where possible with aliases.

I've also added another optional $values parameter to createBlockContent to support tests which were passing these to their own functions, we can then deprecate the $title and $bundle params at a later date if required.

This seems like a good refactor and have no objection to it as another sub-maintainer hat. Not sure if it needs a CR as a new test trait but we'll see!