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>
4c1b6f0476