In anticipation of changing a lot of the focus code, I'm adding some tests for the FocusScope.

As a result, I was able to find and fix a bug where there was an incorrect assert.

I also added some more documentation.

Several of the tests enforce what I think is incorrect behavior related to passing focus off when the widget tree gets rebuilt without focus nodes that were previously there, but I'm not going to change that behavior in this PR.

I also renamed focus_test.dart to focus_scope_test.dart to be more in line with our naming conventions.