test: provide machinery to make and use test_shims

These can be used e.g. to override return values for functions, in
place of the existing scripting of gdb.

This prepends to LD_PRELOAD rather than clobbering it, thanks to a
suggestion from Tomi Ollila.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>