Add a cmake option to toggle schema evolution tests with code generation

[tmadlener]

BEGINRELEASENOTES

• Add a cmake option (PODIO_ENABLE_SCHEMA_EVOLUTION_TESTS) to toggle the schema evolution tests that involve code generation as these more than double the time it takes to run cmake.

ENDRELEASENOTES

[jmcarcell]

I'm not sure what I think about this. On one side, these tests are needed for PRs to be merged but no one will run them (but anyway in the end the ones submitting most of the PRs will be aware...). On the other hands it is a bit annoying to have to generate always all the models every time cmake runs. Alternative options would be not to have to generate the models every time cmake runs and generation being faster (?). Now there is the risk of forgetting to enable these tests in CI or locally. 🤷

[tmadlener]

The "problem" is that usual path for code generation is through the cmake macros and I would like these tests to go through the full machinery just to make sure that that in the end handles things properly. In principle all the generations are entirely independent so it could be quite trivially parallelized if cmake had an easy way to allow for that...

I think turning PODIO_USE_CLANG_FORMAT already makes code generation quite a bit faster, but I don't see an easy other way to speed it up further without heavily reworking things.

We could minimize the risk of forgetting to turn it on by setting this as ON by default, which would give "power users" a way to deliberately turning it off in case these tests are uninteresting or just hinder development for the moment. Additionally, if we add presets (#905), at least it would be a central switch to toggle for all CI and not one that has to be done in N places.

[jmcarcell]

Yes, maybe ON by default is a better default.