Should replace tests by tests around ctest

[sdroege]

See #119 for a start

Without this we currently have no tests at all for the bitfield support, so should fix that as soon as possible.

[ghost]

In the case someone else has time to pick this up, the most recent version is
not in the PR, but at the branch https://github.com/tmiasko/sys/tree/ctest-glib

I would strongly recommend later version that automatically detect incomplete
types, since they require substantial manual work otherwise, or mismatch with
gir generation. My cursory look at constants suggest that they might require
custom support for strings (though, maybe ctest expects them to look different
on Rust side). The pending PR removing special case of enums will be helpful as
well.