before this change, we rely on the default-generated fmt::formatter created
from operator<<, but fmt v10 dropped the default-generated formatter.
in this change, we define formatters for
* std::strong_ordering
* std::weak_ordering
* std::partial_ordering
and their operator<<:s are moved to test/lib/test_utils.{hh,cc}, as they
are only used by Boost.test.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
