Rename GuiTestAssistant and add it to the testing API

[mdickinson]

We should rename the GuiTestAssistant (the one in Traits Futures, not the one in Pyface) to avoid confusion, polish it up, document it, and make it available to users, possibly under a new traits_futures.testing.api module.

See also #252.

Subtasks:

• Rename the GuiTestAssistant: Rename GuiTestAssistant to TestAssistant #388
• Add a minimal set of useful Traits Futures-related methods - most obviously, for waiting on the result of a future.
• Refactor existing tests to use the methods provided by the GuiTestAssistant
• Update documentation on testing recommendations: Document testing strategies and the use of the GuiTestAssistant #220
• Add the GuiTestAssistant to traits_futures.testing.api, and add tests for traits_futures.testing.api.

[kitchoi]

👍 on renaming. The core of this is a dependency of an event loop, in theory, any event loop (e.g. it can be asyncio or tornado event loop in the context of a web server). Reading GuiTestAssistant in a test case that is not targeting GUI raises false alarm.

[mdickinson]

Yes, renaming is a no-brainer. (The existing name is historical: at one point the GuiTestAssistant we were using actually was the Pyface version, then it was a cut-down version of the Pyface version, then a modified cut-down version, extended to cover wx and asyncio, .... It's definitely due a rename.)

Note that there are also some pending changes in #299, which will make the GuiTestAssistant toolkit independent and move the underlying event loop management into a toolkit dependent piece.

[mdickinson]

The renaming has been done. I'm going to defer the rest of this to the next release.