The previous stabilization turned out not to reliably work with FreeBSD
on aarch64 hardware. It was still an improvement overall, but the test
case turned out to be a bit cryptic and when [idle_]send_timeout support
landed in VCL it wasn't obvious how to add coverage.

This attempt bites the bullet and defines one pair of client/logexpect
instances per use case and clearly (I hope) indicates why it is doing
things or not doing them.

Since we now have two clients that aren't expected to complete before
the test case itself the server is in dispatch mode instead of repeating
its scenario. Using barriers in dispatch mode should raise a red flag
for any reviewer, but in this case the barriers outside the server are
properly serialized, are systematically used by every single client, and
as a result should be safe.

As usual, personal testing limited to x86_64 Linux and FreeBSD.