`git subtree` to import re-used testing code from `nidm` repository

Following discussions at incf-nidash/nidm#124, this pull request implements the re-use of common testing code (and files) from the incf-nidash/nidm repository using `git subtree`. 

This PR replaces #4 to remove the history of nidm repository (described to be a common practice in [Alternatives To Git Submodule: Git Subtree](http://blogs.atlassian.com/2013/05/alternatives-to-git-submodule-git-subtree/)).