run unit-test in build scripts

[rtbo]

Describe the problem you are trying to solve
I have quite complex build scripts that require some unit-testing.

Describe the solution you'd like
I'd like cargo test to also run #[test] defined in build script files.

[epage]

A workaround is to pull the build script logic out into a library that the build script then calls. This would be made better with rust-lang/rfcs#3452.

The challenge to this is we'd need to elevate build scripts to being a dedicated target while right now it is just a field in the package.

[weihanglo]

Or make the custom build logic into a workspace member, and use it as a “build dependency”. The downside is now you need to publish your build script to crates.io.