Add "UI" tests (trybuild) to check macro output/errors

[jsdw]

Let's have some specific tests to check that the macro works or not, and that the errros we get back are what we expect!

[jsdw]

I added a framework for adding UI tests (and a single test to check a fix) in #567, so we can build on that to test some other metadata codegen edge cases.

[jsdw]

I'm going to close this; we have a framework and are adding tests to it now when it makes sense.