the operator<< for `cql3::expr::test_utils::mutation_column_value` is
preserved, as it used by test/lib/expr_test_utils.cc, which prints
std::map<sstring, cql3::expr::test_utils::mutation_column_value> using
the homebrew generic formatter for std::map<>. and the formatter uses
operator<< for printing the elements in map.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>