[Bug libstdc++/108062] Test spdlog c++20 std::format

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108062

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Why am I having to say this AGAIN?

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107886#c7
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107886#c9
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104592#c3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104592#c5