tests/README

Improvement to 14ce129cfcd: Use try/except and pass filename on errors

Without the second part, the error message would be
  abort: Is a directory
instead of
  abort: Is a directory: /home/user/.cvspass

To run the tests, do:

cd tests/
python run-tests.py

See http://www.selenic.com/mercurial/wiki/index.cgi/WritingTests for
more information on writing tests.