Closes a test-coverage gap from #5421: ordered_json (and the alt_string-based
basic_json specialization from unit-alt-string.cpp) were never round-tripped
through the binary formats (CBOR/MessagePack/UBJSON/BSON/BJData), nor through
flatten()/unflatten(), diff()/patch()/patch_inplace(), or merge_patch(). Also
adds a std::formatter<ordered_json> spot-check, mirroring the precedent set
by the format_as() ADL-deduction test.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>