Check results in tests

[certik]

And in a test like this one: https://github.com/gxyd/c_mpi/blob/main/tests/allreduce.f90, we need to check the result to ensure that:

• It is computing the correct result
• It is computing exactly the same answer with any MPI ranks (we'll only test 1, 2, and 4 at the CI).

[certik]

Check the result like we do in integration test: "if (abs(result - 55.47) > eps) error stop"

Make sure to print the result too with full double precision accuracy, it should differ as you run it with different MPI ranks within good (machine) accuracy.