[MERGE] More than double the speed of "bzr selftest"

[Ian Clatworthy]

John Arbash Meinel wrote:

> Ian- do you want this for 0.17? As a change to the test infrastructure I
> consider it safe enough, but that is my personal opinion.

I appreciate that there is very low risk of it breaking the code itself.
But I feel it is safer to leave the test infrastructure unchanged this
week. Let's delay merging this until the start of 0.18 please.

Ian C.