No tests for wrapper correctness

[melonedo]

Currently, we only have tests for the integrity of the generated wrapper, while correctness needs to be tested on downstream packages.
Maybe we could build a custom test library in C (with BinaryBuilder), then relevant tests can be hosted on Clang.jl itself.

[Gnimuc]

+1 for this.

There were some test cases before. I removed them because all of these test cases were too trivial.

For new features like bitfield, we do need decent unit tests. It's not easy to work with BB2 when developing the package interactively. It turns out this is probably an ideal use case for ClangComplier.jl with which we can compile the C code within Julia and call the library through LLVM.jl.