Update xmltest.cpp

Update xmltest.cpp

In PrintString, move the optimization under the if ( _processEntities ) block and retain the strlen Write in the new else block. Added a test case.

* clarify what is being tested to avoid duplicate lines in output
* prevent optimizations based on undefined behavior

The original name is to general. This prevents possible name collisions with other defines, enums, etc. from projects where tinyxml2 is used.