[libc++][test] Move format.functions ASCII tests to `libcxx/test/libcxx` (#78661)

As @cpplearner explained in microsoft/STL#4328:

> libc++'s "ascii" mode (controlled by the `_LIBCPP_HAS_NO_UNICODE`
> macro) means "every code unit outside ASCII is treated as a valid
> printable character". AFAIK we \[MSVC's STL\] don't support such a mode.

Because these files are testing a non-Standard mode, they should be
moved from `libcxx/test/std` to `libcxx/test/libcxx`.