Add clarifying statement to Abort docstr.

Add a statement to the Abort docstr indicating that only an info level
log message will be output upon Abortion.
In test_Abort, add checks on logging and start

In the test_Abort test, I added a check to ensure that Async.start() is
not called. Also, I added checks on logging.info() and logging.error()
to ensure that info is called and error is not.