Some more editor tests, primarily dealing with programmatically created editors

- Open up an arbitrary number of editors on either of the two source data areas
- Open up multiple editors on the same source area, changes to one editor, should be reflected everywhere that source data is opened
- Closing an editor (which destroys it) shouldn't throw an error when a new one is created

Edit Content A
this is the first content area holding some data
Edit Content B
this is the second content area holding some data