[MERGE] Move the notes on writing tests out of HACKING into a new file, and improve them.

[Andrew Bennetts]

Robert Collins wrote:
> On Wed, 2008-08-13 at 08:53 +1000, Andrew Bennetts wrote:
> > This change moves the notes on writing tests out of HACKING into a new file, and
> > improves them.
[...]
> 
> Its a little hard to tell:
>  - what you moved
>  - what you changed

Yeah.  I did consider trying to present this work in smaller steps, but I
touched most of the text (even if only a little bit) so it was hard to do
anything other than one big diff.  I did go through several iterations before
making this commit, but if I had committed and submitted the iterations
individually it would have been pretty noisy because a lot of text moved
back-and-forth in the file several times.

Also, I think this document had previously suffered from not being looked at as
a whole for quite some time, so it's probably a good thing that it is read from
start-to-finish.  It does put a fairly large burden on the reviewer, though :(

-Andrew.