message: tests: Expand write_flush coverage
  Adjust the buffer size to be right on the edge of the overflow, make
  sure we actually output &quot; instead of ".
diff: @@ -645,20 +645,23 @@ TEST(write_flush_coverage)
  // this creates a node that uses short sequences of lengths 1-6 for output
  xml_node n = doc.append_child(STR("n"));
  - n.text().set(STR("<&\""));
  - n.append_child(node_comment);
  xml_attribute a = n.append_attribute(STR("a"));
  - size_t basel = save_narrow(doc, 0, encoding_auto).size();
  - size_t bufl = 10240;
  + xml_attribute b = n.append_attribute(STR("b"));
  + b.set_value(STR("<&\""));
  +
  + n.append_child(node_comment);
  +
  + size_t basel = save_narrow(doc, format_raw, encoding_auto).size();
  + size_t bufl = 2048;
  for (size_t l = 0; l <= basel; ++l)
  {
  std::basic_string<pugi::char_t> pad(bufl - l, STR('v'));
  a.set_value(pad.c_str());
  - std::string s = save_narrow(doc, 0, encoding_auto);
  + std::string s = save_narrow(doc, format_raw, encoding_auto);
  CHECK(s.size() == basel + bufl - l);
  }
  }
  