hugr-llvm: Export testing utilities

[doug-q]

Once #1601 is merged:

hugr-llvm has some useful testing utilities in src/test.hs and src/emit/test.rs. In moving the lowering for the rotation extension to tket2 in CQCL/tket2#697 we removed some tests because they could no longer access those testing utilities.

Let's export these tools from hugr-llvm so that downstream crates can benefit from them. Being test code, it's not as polished as we would want, so a bit of tidying/renaming/documenting will be required. We can guard these tools with a feature if there is a benefit in doing so(i.e. making some dependencies optional).

We could also put the code in a new crate (say hugr-llvm-test) and add that crate as a dev-dependency to hugr-llvm. This would be a circular dependency (hugr-llvm -> hugr-llvm-test -> hugr-llvm), but this is allowed as one of the edges is a dev-dependency edge. It's not clear to me what the benefit of this would be.

[ss2165]

hugr-llvm is under the llvm hugr feature, I suggest exporting these under a new llvm-tests feature with all the test dependencies