Improve test names
Rework setup code to more easily support more tests

Instead of returning details only about one of the three created buffers,
return details about all three.
More tests for :buffer ex command
Remove some more :buffer incompatibilities with vim

The :buffer command now

 1. Behaves correctly when a bang is provided or not.
 2. Accepts the buffer command as a count.
Merge pull request #546 from benarmston/vim2-more-tests

Improve compatibility for :buffer command and add tests for existing behaviour