run-tests.sh: Ignore user's GREP_OPTIONS

My .bashrc sets `GREP_OPTIONS=--exclude=TAGS`, which was causing
`assert_tag` to always fail.
Ctags (Vi) format

TODO:

* Add tests (figure out how to get vi to output filename & line number
  to stdout, for integration tests).
* Sort the tags?
Properly reference the python2 executable

See here: http://legacy.python.org/dev/peps/pep-0394/
Ignore symlinks
Change comment that breaks vim highlighting
Sketch of assert_vim
Add some vim tests

This commit adds assert_vim tests to some of the existing tests that
call assert_emacs.