cleanup delete tests

[ianstormtaylor]

Do you want to request a feature or report a bug?

Debt.

What's the current behavior?

There are lots of tests for editor.delete() right now that are poorly named, or that contain too much extraneous stuff in the test cases's document that isn't directly related to the specific behavior being tested.

It makes it hard to reason about what exactly is failing when a test fails. They should be cleaned up to have a consistent naming scheme. And remove any extra nodes/text that isn't necessary for each test's behavior.

[ianstormtaylor]

I believe that this may be fixed by #3093, which has changed a lot of the logic in Slate and slate-react especially. I'm going to close this out, but as always, feel free to open a new issue if it persists for you. Thanks for understanding.