some smallish parallel ctest improvements

the goal is to make it faster on computers with many cores: The
easiest way to do this is to ensure that the longest running tests are
not taking too much time and that they need about the same time. Thus
this patch contains the following changes which limits the CPU time
taken by each test to about two minutes in debug mode on my machine:

 - the water-air problem using the non-isothermal primary variable
   switching model now uses an 16x16 instead of a 32x32 grid. as a
   compensation it now runs for a year instead of 5000 seconds and the
   global grid refinement is now tested.
 - the end time of the lens problem ctests is now 3000 instead of
   30000 seconds. The binary itself does not change at all.
 - sort the tests in the CMakeLists.txt roughly in the order of their
   required time. (this will cause ctest not having to wait for long
   running test which were started late for too long.)