(so that stale test files, which can be very large, get wiped out)

Patch by Ben Hayden.

concurrent.futures.

whole HTTP exchange, not only the opening.

The options listed in the table are now links to their documentation,
and the table uses compact markup to make it easier to read and edit.
First proposed in #10998.

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.

Algorithm and initial patch by Michael Henry.