Integration tests migration (final)

With this PR, all integration tests covered by [SCC-merge](http://hudson.openmicroscopy.org.uk/view/Mgmt/job/SCC-merge/) should be integrated under `test/`.

Consequences of these changes is that:
- all tests are now run for every Pull Request by the Travis build (except tests pushing back to Github)
- the [SCC-merge](http://hudson.openmicroscopy.org.uk/view/Mgmt/job/SCC-merge/) job and [SCC-self-merge](http://hudson.openmicroscopy.org.uk/view/Mgmt/job/SCC-self-merge/) jobs can now be unified into a single test job running all sec tests (including the ones pushing back to Github)
- all the testing infrastructure for scc is now self-contained in the repository itself

Additional commits fixed in the scope of this PR;
- fixed `parse_body()` for PRs with empty bodies (on snoopys-sandbox)
- reuse `parse_pr()` method in `UnrebasedPRs`
- refactor `CheckMilestone` command to inherit from `GitRepoCommand`