ollie.txt

I've done something like this in the past: I installed the test executables 
somewhere in $out (although presumably one could use an output different 
than $out). You will need to change `testTarget` and/or `checkPhase` as well, 
so as to prevent the tests from being run with Nix. Then, once the thing is
installed, you just execute /nix/store/whatever/libexec/tests/foo-tests. 

postInstall = ''
  if [ "$doCheck" = "1" ]; then
    mkdir -p $out/libexec/tests
    cp ./dist/build/foo-tests $out/libexec/tests/foo-tests
  fi
'';