Always setup (DY)LD_LIBRARY_PATH for testsuite

Now that Cabal is in charge of RPATH handling on certain OS',
we must always setup a correct (DY)LD_LIBRARY_PATH when
running the testsuite. Not just when we are building relocatable
packages.

The "problem" is, is that Cabal now adds an RPATH pointing
to the installation location of the library. However, during
testing, the library won't be there yet. We much hence setup
a (DY)LD_LIBRARY_PATH that includes the dist/build dir.