[libc++][test] Don't include `test_format_context.h` in `parse.pass.cpp` (#83734)

The `parse.pass.cpp` tests doen't need to call
`test_format_context_create` to create a `basic_format_context`, so they
shouldn't include `test_format_context.h`.

The `to_address` mechanism works around the iterator debugging
mechanisms of MSVC STL. Related to
[LWG3989](https://cplusplus.github.io/LWG/issue3989).

Discovered when implementing `formatter<tuple>` in MSVC STL. With the
inclusion removed, `std/utilities/format/format.tuple/parse.pass.cpp`
when using enhanced MSVC STL (and `/utf-8` option for MSVC).