Adding more CXX20 testing in CI.

[lemire]

We may want to turn FASTFLOAT_CONSTEXPR_TESTS ON by default when C++20 is detected.

[lemire]

@leni536 Thoughts on the failing tests ?

[leni536]

https://github.com/fastfloat/fast_float/actions/runs/4521288444/jobs/7962950990#step:3:33

Looks like gcc 9.4.0 doesn't have the three C++20 features that we are testing for, even though it allows building in C++20 mode (possibly with -std=c++2a flag in that version, CMake happily sets that).

So I wouldn't enable constexpr testing in that workflow.

Further errors are less relevant, but they show that several other C++20 constexpr features are missing from that compiler, like allowing uninitialized variables.

[lemire]

@leni536 Fantastic. I had not realized at once that it was GCC 9. Fixing.