Support testing GMX_BUILD_MDRUN_ONLY

Testing that CMake option requires that we do not call the
regression tests, since they require more than mdrun to be
built. Created a flag to implement that, in case of future
conditions that might also want to suppress running the
regressiontests.

Change-Id: Ice1b53b0b6b77f8505d52e1f5edb794d1bb2abf8
Merge "Support testing GMX_BUILD_MDRUN_ONLY" into 5.0.0