This patch doesn't quite fix the 'run in a VM with Samba share'
timing problem, but it should at least make it better.

Why I consider this a bug rather than an API change: the API change was
to Message, which didn't used to return Headers unless you added them
yourself.  Now it does (for 8bit binary header input), so decode_header
needs to be able to handle them.

few other assertEqual tests that snuck in), and expand the docstrings and
comments explaining why and how these tests are supposed to work.

Patch by Sandro Tosi.

Patch by Sandro Tosi.  The main purpose of this option is to allow
an alternate set of tests files to be used when running tests
of the regrtest tool itself.

Algorithm and initial patch by Michael Henry.