test_mem_leak.sh: add convenience script for running valgrind

test_mem_leak.sh: add convenience script for running valgrind

This script will run valgrind against check_mem_leaks, and
determine if there are any memory leaks.

This is not run as part of "make check", as valgrind really only works
on GNU/Linux. (It is available on OSX, but does not really work).

We will need to run this ourselves (or from our continuous integration
server) to periodically check for leaks.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@902 64e312b2-a51f-0410-8e61-82d0ca0eb02a