Add test for opening a note by key

Required corresponding change to simplenote.vim so when running in test
environment it opens in current buffer.

Since total number of notes created has changed, need to change
assertion for list of notes.

References #17
Minor change to where test comments are placed

Rather than place at first execution block for that test, place in most
relevant block.

References #17
Add local cleanup task to tests

Remove all unnecessary buffers. And with that:

Fixes #17