Refactor mdrun integration tests

Future work will need the ability for such a test to call mdrun more
than once and compare the results. This means that a test fixture
object needs to be able to keep track of the separate files in an
orderly way. Created SimulationRunner to manage the setup and calls to
grompp and mdrun, somewhat like an implementation
class. MdrunTestFixture now owns a single such object. Created
MdrunTestFixtureBase to be the common point for managing issues of
controlling the execution environment of mdrun, so that future work
can derive classes from it that use more than one SimulationRunner.

Change-Id: Ic2ab82b3bacb01f9bf77de67909567352fc2d306