[CI] Fix clang++-15 regression test job

[bluetarpmedia]

Updated expected results for clang++-15 on ubuntu-latest.

[hsutter]

Thanks! But a question: Clang-15 no longer works with formatted interpolation?

[bluetarpmedia]

Thanks! But a question: Clang-15 no longer works with formatted interpolation?

That's because it's now using the libstdc++ from gcc 12. Previously it was using libstdc++ from gcc 13. So it's not really a clang compiler issue but the version of the standard library that it uses.

Something changed with the GitHub Action ubuntu images which means gcc 13 is no longer available on ubuntu-latest, so clang now picks up the highest version of gcc that is installed (12) and uses that version of libstdc++.

I figured the best option for now is to fix this regression test job, but I've made a note to improve things:

1. Stop using ubuntu-latest since over time this "pointer" will change, and we won't have consistent results. Instead I think it's better to use explicit versions

2. I think we need to add tests for clang and libc++ (the LLVM standard library), instead of (or in addition to) clang with libstdc++.

EDIT - Just realised that 2nd case is covered already on macOS, but not ubuntu. So maybe not as important as I thought.

[hsutter]

Sounds good, thanks for paying attention to this! Appreciated.