Because -rpath/-R may have been used, our settings of LD_LIBRARY_PATH

Because -rpath/-R may have been used, our settings of LD_LIBRARY_PATH
and friends may be entirely useless.  In such a case, LD_PRELOAD is
the answer, at least on platforms using LD_LIBRARY_PATH.  There might
be other variables to set on other platforms, please fill us in...

For now, we only do this with the tests, so they won't fail for silly
reasons like getting dynamically linked to older installed libraries
rather than the newly built ones...

PR: 960