Remove mpiexec in cmake test target

[jhale]

Exporting a test target is nice but the fixed mpiexec hits walls quickly - we have no idea what downstream consumers need to use.

[schnellerhase]

Looks good to me. Hiding the parallel nature of execution from the caller, might also cause more problems with ctest's own parallelisation options.

[jhale]

By colleagues here at uni.lu I've been persuaded that we should use the output of FindMPI to discover the correct way to invoke mpiexec:

https://cmake.org/cmake/help/latest/module/FindMPI.html

find_package(MPI)

if(MPI_FOUND)
  execute_process(
    COMMAND
      ${MPIEXEC_EXECUTABLE}
      ${MPIEXEC_NUMPROC_FLAG}
      ${MPIEXEC_MAX_NUMPROCS}
      ${MPIEXEC_PREFLAGS}
      <executable>
      ${MPIEXEC_POSTFLAGS}
      <args>
  )
endif()

[jhale]

Partial reversion to old behaviour completed in #3984.