Don't build tests until they are run

There is no advantage to normal users, to build the tests until the
user decides to run them. Also avoids that the user thinks that
Gromacs doesn't compile on his platform if there is only a problem
with the testing framework.

Added a GMX_DEVELOPER_BUILD option which restores the old behavior.

Change-Id: Ifda2184eb45bf58b8798f21a29aea19bef57c690