since Boost.Test relies on operator<< or `boost_test_print_type()`
to print the value of variables being compared, instead of defining
the fallback formatter of `boost_test_print_type()` for each
individual test, let's define it in `test/lib/test_utils.hh`, so
that it can be shared across tests.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#18260