v3 of test speedup by replacing gdb with LD_PRELOAD

[Daniel Kahn Gillmor]

Here are the two patches from this series, revised according to the
on-list discussion.

In particular, the revision from Bremner's original series are:

 * LD_PRELOAD is now prepended to, not clobbered (thanks, Tomi!)
 * gen_insert_msg is not moved

We still don't deal with platforms that don't support LD_PRELOAD (like
MacOS), but i'd encourage us to take Ralph at his word and go ahead
with these changes now, cleaning them up on those platforms later.

     --dkg