Add tests for the C interfaces

[amontoison]

As suggested by Nick in #63, we should add some unit tests in C.
It would be a good sanity check for the interfaces in high-level languages like MATLAB, Python and Julia.
It needs to match the functionality of utest.F90 and ctest.f90 in src/test.

[nimgould]

I've completed @amontoison 's unconstrained C tests, and added constrained ones. I am also underway with the 0-based, row-centric C versions, and this shouldn't be hard to do (just needs a bit of time)

[amontoison]

Thanks Nick! I juqt uncommented the related tests in test/meson.build to check if it works on all platforms.