VOL-TESTS

Merge pull request #17 in HDF5VOL/vol-tests from ~JHENDERSON/vol-tests:master to master

* commit 'd218fe4ae97a5509390007c4e5996882c9fbb883':
  Add external link iteration and visiting tests
  Remove some TODO comments
  Add tests for recursive link iteration with cyclic links
  Update link copying and moving tests to not write links to root group
  Add H5Literate(_by_name) 'only hard links' and 'only soft links' tests
  Split up link iteration tests into several parts