src/config/rcxml.c: parse xml from buffer

Avoid unit tests writing to/from files by using xmlParseMemory() instead
of xmlReadFile().

tests/t1000-rcxml-simple-parse.c

@@ -33,4 +33,6 @@ int main(int argc, char **argv)
 
 	diag("Simple parse rc.xml");
 	ok1(rc.client_side_decorations);
+
+	return exit_status();
 }