test/lib: include <fmt/std.h> for formatting std::optional
before this change, when compiling with fmtlib v11.0.2 and clang
v19.1.0, the compiler fails like:
```
/usr/bin/clang++ -DBOOST_REGEX_DYN_LINK -DBOOST_REGEX_NO_LIB -DBOOST_UNIT_TEST_FRAMEWORK_DYN_LINK -DBOOST_UNIT_TEST_FRAMEWORK_NO_LIB -DDEBUG -DDEBUG_LSA_SANITIZER -DFMT_SHARED -DSANITIZE -DSCYLLA_BUILD_MODE=debug -DSCYLLA_ENABLE_ERROR_INJECTION -DSEASTAR_API_LEVEL=7 -DSEASTAR_DEBUG -DSEASTAR_DEBUG_PROMISE -DSEASTAR_DEBUG_SHARED_PTR -DSEASTAR_DEFAULT_ALLOCATOR -DSEASTAR_LOGGER_COMPILE_TIME_FMT -DSEASTAR_LOGGER_TYPE_STDOUT -DSEASTAR_SCHEDULING_GROUPS_COUNT=16 -DSEASTAR_SHUFFLE_TASK_QUEUE -DSEASTAR_SSTRING -DSEASTAR_TYPE_ERASE_MORE -DXXH_PRIVATE_API -DCMAKE_INTDIR=\"Debug\" -I/home/kefu/dev/scylladb -I/home/kefu/dev/scylladb/build/gen -I/home/kefu/dev/scylladb/seastar/include -I/home/kefu/dev/scylladb/build/seastar/gen/include -I/home/kefu/dev/scylladb/build/seastar/gen/src -I/home/kefu/dev/scylladb/build -isystem /home/kefu/dev/scylladb/abseil -isystem /home/kefu/dev/scylladb/build/rust -g -Og -g -gz -std=gnu++23 -fvisibility=hidden -Wall -Werror -Wextra -Wno-error=deprecated-declarations -Wimplicit-fallthrough -Wno-c++11-narrowing -Wno-deprecated-copy -Wno-mismatched-tags -Wno-missing-field-initializers -Wno-overloaded-virtual -Wno-unsupported-friend -Wno-enum-constexpr-conversion -Wno-unused-parameter -ffile-prefix-map=/home/kefu/dev/scylladb/build=. -march=x86-64-v3 -mpclmul -Xclang -fexperimental-assignment-tracking=disabled -Werror=unused-result -fstack-clash-protection -fsanitize=address -fsanitize=undefined -MD -MT test/lib/CMakeFiles/test-lib.dir/Debug/cql_assertions.cc.o -MF test/lib/CMakeFiles/test-lib.dir/Debug/cql_assertions.cc.o.d -o test/lib/CMakeFiles/test-lib.dir/Debug/cql_assertions.cc.o -c /home/kefu/dev/scylladb/test/lib/cql_assertions.cc
In file included from /home/kefu/dev/scylladb/test/lib/cql_assertions.cc:12:
In file included from /usr/include/fmt/ranges.h:20:
In file included from /usr/include/fmt/format.h:41:
/usr/include/fmt/base.h:2673:45: error: implicit instantiation of undefined template 'fmt::detail::type_is_unformattable_for<std::vector<std::optional<seastar::basic_sstring<signed char, unsigned int, 31, false>>>, char>'
2673 | type_is_unformattable_for<T, char_type> _;
| ^
/usr/include/fmt/base.h:2735:23: note: in instantiation of function template specialization 'fmt::detail::parse_format_specs<std::vector<std::optional<seastar::basic_sstring<signed char, unsigned int, 31, false>>>, fmt::detail::compile_parse_context<char>>' requested here
2735 | parse_funcs_{&parse_format_specs<Args, parse_context_type>...} {}
| ^
/usr/include/fmt/base.h:2884:47: note: in instantiation of member function 'fmt::detail::format_string_checker<char, int, std::vector<std::optional<seastar::basic_sstring<signed char, unsigned int, 31, false>>>, std::vector<std::optional<managed_bytes>>>::format_string_checker' requested here
2884 | detail::parse_format_string<true>(str_, checker(s));
| ^
/home/kefu/dev/scylladb/test/lib/cql_assertions.cc:132:34: note: in instantiation of function template specialization 'fmt::basic_format_string<char, int &, std::vector<std::optional<seastar::basic_sstring<signed char, unsigned int, 31, false>>> &, const std::vector<std::optional<managed_bytes>> &>::basic_format_string<char[35], 0>' requested here
132 | fail(seastar::format("row {} differs, expected {} got {}", row_nr, row, actual));
| ^
/usr/include/fmt/base.h:1616:8: note: template is declared here
1616 | struct type_is_unformattable_for;
| ^
/home/kefu/dev/scylladb/test/lib/cql_assertions.cc:132:34: error: call to consteval function 'fmt::basic_format_string<char, int &, std::vector<std::optional<seastar::basic_sstring<signed char, unsigned int, 31, false>>> &, const std::vector<std::optional<managed_bytes>> &>::basic_format_string<char[35], 0>' is not a constant expression
132 | fail(seastar::format("row {} differs, expected {} got {}", row_nr, row, actual));
| ^
```
because the formatter for `std::optional<>` is defined in fmt/std.h.
so, in this change, we include the used header.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#20922
9b5eab0dde