Easy optimisation (turned off node self-tests)

As an easy step towards "optimising" Conglomerate I committed a one-line
change that turns off self-testing.

In src/cong-node.h there's a define of CONG_NODE_SELF_TEST which either
runs a batch of tests on a node, or collapses away at preprocessing
time.  This macro gets used before and after every atomic modification
within a command, so it gets used a _lot_.  It also gets used whenever
you load a new document.

The self-tests haven't failed for a while for me, and there feels like a
noticeable speedup, so I thought we'd turn them off for now.  In
particular, there's no longer such a scary delay when you load a large
document.  

If you're hacking on the code, you may want to turn the selftests back
on, especially if you're working with the CongCommand system (or the
systems below it).

If only all optimisations were as easy...

Dave