This variable is set to allow the use of the nice ParseAndAddCatchTests script
in the case where a launcher is needed to execute the script.
This is introduced to allow to launch unit tests using mpi. In this case one can
write for instance
set(OptionalCatchTestLauncher ${MPIEXEC} ${MPIEXEC_NUMPROC_FLAG} ${NUMPROC})
before calling the ParseAndAddCatchTests function.
This might prove helpful when the package managers either doesn't
have Catch at all, or provides it in obsolete version (Ubuntu 16.04,
I am looking at you).
Closes#1383
