test: define a more generic boost_test_print_type

fmt::is_formattable<T>::value is false, even if

* T is a container of U, and
* fmt::is_formattable<U>, and
* U can be formatted using fmt::formatter

so, we have to define a more generic boost_test_print_type()
for the all types supported by {fmt}. it will help us to ditch the
operator<< for vector and unordered_map in Seastar, and allow us
to use the fmt::formatter specialization of the element
types.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

test/lib/test_utils.hh

@@ -113,10 +113,16 @@ extern std::mutex boost_logger_mutex;
 
 }
 
+namespace internal {
+
+template <typename T, typename Char = char>
+concept formattable = fmt::is_formattable<std::remove_reference_t<T>, Char>::value;
+
+}
+
 namespace std {
 
-template <typename T>
-requires fmt::is_formattable<T>::value
+template <::internal::formattable T>
 std::ostream& boost_test_print_type(std::ostream& os, const T& p) {
     fmt::print(os, "{}", p);
     return os;