start to replace dependence of test suite on gdb with LD_PRELOAD shims

[David Bremner]

These patches are somewhere in between WIP and proposed for
merging. I'm sure there are some shell scripting tweaks needed, and
it's a potentially disruptive enough change that I want to wait until
after 0.29.  On the other hand, it works for me, and the API seems
like an improvement on what it replaces.  I'm particularly interested
in the portability of this approach.