Improve testsuite

[Meijuh]

From TODO file in testsuite/ :

The test suite needs still to be extended with:

tests for the distributed tools using --mpi, --workers and --procs. The distributed backend is tested in alg_backends__, but without parallelism. See also the HAVE_POSIX flag in Makefile.am A test case which does (parallel) LTS construction, reduction comparison. See lts_generation tests and mini tutorial in main LTSmin manual. Optional multi-core backend tests. See HAVE_ATOMICS in configure.ac More fine-grained dependencies. See Makefile.am, e.g. lts_generation can be executed partly without CADP or Mcrl2. Integration of src/tests/test-_ binaries POR tests DFS_FIFO tests