message: tests: Add xml_buffered_writer coverage test
  This test triggers flush() condition for each optimized write() method.
diff: @@ -639,6 +639,30 @@ TEST_XML_FLAGS(write_roundtrip, "<node><child1 attr1='value1' attr2='value2'/><c
  }
  }
  +TEST(write_flush_coverage)
  +{
  + xml_document doc;
  +
  + // 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;
  +
  + 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);
  + CHECK(s.size() == basel + bufl - l);
  + }
  +}
  +
  #ifndef PUGIXML_NO_EXCEPTIONS
  struct throwing_writer: pugi::xml_writer
  {
  