Some flakes are caused by weird races in this code. Plus, if we
get things to write straight to files, we might see things in
there on post-mortem which happen after the python runner exits.
It's a bit less efficient, but much simpler. Let's see if it helps!
Some tests need a rework now, since we don't get a failure (except
eventual timeout), but they're simpler.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>