Use git snapshots of the tests if are on git

* Snapshots are available from http://repo.or.cz/w/gromacs
  but hidden by forwards from http://gerrit.gromacs.org/snapshot
* Discussion on I7ba82b22a75532516f481dcb5e4a870047c0bfe8

Change-Id: If3d00d2de8d7286c3b5d7f4b67d468b8d628e28f
Merge "Use git snapshots of the tests if are on git" into release-4-6