tests cleanup

. make common.o link with the tests instead of being
	  #included as common.c
	. fix warnings about missing prototypes by declaring functions
	  static
	. reduces some duplicated code

Change-Id: Ic2a765d7f5886add5863190efec3fdd2d2ea2137

test/test61.c

@@ -5,8 +5,9 @@
 #include <stdio.h>
 #include <unistd.h>
 
-#define MAX_ERROR 5
-#include "common.c"
+int max_error = 5;
+#include "common.h"
+
 
 void dangling_slink(int sub_test, char const slink_to[PATH_MAX]);