Function to_string

src/test/common/test_time.cc:204–209

namespace {
  template<typename Rep, typename Period>
  std::string to_string(const chrono::duration<Rep, Period>& t)
  {
    std::ostringstream ss;
    ss << t;
    return ss.str();
  }

  void float_format_eq(string_view lhs,
                       string_view rhs,