Tests: Stabilize include order in StringFileTest

This test generates a header file which is not self-contained.
Include it in a separate block of includes so that tools that
sort includes do not move it.

Tests/StringFileTest/StringFile.cxx

 #include <stdio.h>
 #include <string.h>
+
 #include "OutputFile.h"
 
 int main(int, char*[])