Tests: make mpiexec and numdiff optional

Tests: make mpiexec and numdiff optional

Making numdiff (and mpiexec) optional has a number of advantages:

 - First of all, we can call deal_ii_pickup_tests() in user projects
   unconditionally. Previously, it was always necessary to add some
   CMake code to only call the macro if numdiff was available. Or,
   otherwise to frustrate everyone who just wants to configure and
   compile your project and doesn't happen to have numdiff installed.

 - This refactoring will allow us to simply call deal_ii_pickup_tests()
   for configuring out quick tests.