make tests parallel

I've upgraded blead to Test::Harness 3.13, which has the ability to run tests
in parallel (and make CPU fans spin faster)

Right now, of course, the existing test harness scripts don't run the core's
tests in parallel. Which is a shame, as we know that the tests will complete
much more quickly if they are run in parallel.

Is anyone interested in working on getting the cores tests running in
parallel? I believe that Andy Armstrong has a pretty functional prototype
that will run discrete directories of tests in parallel, which is a good
first step, so you wouldn't be starting from scratch.

What's needed is

* get that merged cleanly with t/harness in core
* figure out the best way to invoke parallel tests from the Makefile


With that in place we can all work on identifying which tests use
unimaginative temporary file names, or other things that conflict, and
prevent them from running in parallel. As we incrementally fix those, we can
achieve better parallelism.

Nicholas Clark