Added sharedarray unlink tests. Minor cleanup in sharedarray.

The SharedArray constructor is expected to unlink the shmem segment and also close the `fd` from `shm_open` immediately after the segment has been mapped on all participating processes. This ensures an automatic release of the segment when the last process has unmapped the same.

This PR adds a test on Linux that tests for the non-existence of the shmem segment under `/dev/shm` after the construction. There does not seem to be an equivalent test under OSX.