Fix missing returns in the Doxygen test suite code.

Missing returns resulted in test suite failures now that it is built with
-Wreturn-type, so avoid them by simply making most of the functions void and
adding explicit return statements to a couple of them that used Doxygen
\return command in their documentation, which doesn't make sense for the void
functions.

Examples/test-suite/python/doxygen_translate_all_tags_runme.py

@@ -262,13 +262,13 @@ Remarks: Some remark text
 
 Another remarks section
 
-:rtype: int
+:rtype: void
 :return: Whatever
 
-:rtype: int
+:rtype: void
 :return: it
 
-:rtype: int
+:rtype: void
 :return: may return
 """)