Don't remove /tmp/concat in after :each

If the spec files are run in a random order, removing /tmp/concat after
certain tests could cause the next tests to fail if they do not recreate
this directory.

(cherry picked from commit 1edafdcf42c08e1c434d51c1cdc523fca612e4d3)
Merge pull request #144 from hunner/fix_rm2

Don't remove /tmp/concat in after :each