Test the output of "gmx mdrun help"
As we migrate mdrun to use the gmx::Options framework, several chunks
of text and many option descriptions will move to their respective
modules. However users would prefer the output of "gmx mdrun help" to
only change in ways that we as developers expect.
This will also provide a stable path towards e.g. implementing gmx
minimize, gmx tpi, gmx rerun, etc. and having the help text relevant
only to that runner. It may also be useful if in future we structure
mdrun modules to write their help via e.g. "gmx help mdrun pulling".
Change-Id: Iccd07f797004d8ff24eb56ca528a3fe9148b6749