Have test file add its own directory to load path

This makes it so that one can do "emacs -batch -l async-test.el ..."
without having to also add "-l async.el -l async-file.el" at the
command line.
Add more comprehensive testing for anti-closure feature

Based on suggestions in
https://github.com/jwiegley/emacs-async/pull/18#issuecomment-26665920
More robust evaluation and closure-protection of start-func