Fix test in case of non empty env.

In some cases where environment is not empty (e.g. in a sandbox) tests fail with:

1) Data.Conduit.Process.Unix.unix-process-conduit environment is set
expected: "foo=bar\n"
 but got: "LD_PRELOAD=libsandbox.so\nfoo=bar\n"

This patch fixes issue as in filter value we are interested in.