Constraints, rendered as static_asserts are not tested properly

[steve-downey]

Need to add tests that fail to compile.
q.v.
https://ibob.bg/blog/2022/10/04/testing-build-failure-with-cmake/
https://stackoverflow.com/questions/30155619/expected-build-failure-tests-in-cmake/50665823#50665823
https://gist.github.com/tim-welch/00e07dc57864de36739945156e4049e2
https://stackoverflow.com/questions/24692820/how-to-test-the-static-assert-in-my-library-with-cmake

Some of the docs linked mention working around old cmake issues, which we should not do.

The approach in ibob.bg looks somewhat compatible with how lit is working, in that it's confirming the reason for failing to compile.