Do not use `ctest` anymore, tests are now launch directly (manually)
using generated unit tests binaries (unit_tests and mpi_unit_tests)

`ctest` can always be invoked but not using `make test`.

The reason for this change is that `ctest` runs a binary for each test
which is not what we want now. The goal of this development is to
provide a mesh database that is built **once** for all: the cost of
building meshes could be prohibitive and their multiplication should
be avoided for obvious performances and storage reasons.