tests CHANGE headers includes cleanup

Set necessary include directories instead of including library headers
in tests via relative path (which cause problems when building in
some nonstandard build directories).
diff --git a/tests/utests/test_set.c b/tests/utests/test_set.c
index 30a775d..d53d7c2 100644
--- a/tests/utests/test_set.c
+++ b/tests/utests/test_set.c
@@ -22,7 +22,7 @@
 
 #include <cmocka.h>
 
-#include "../../src/set.h"
+#include "set.h"
 
 #define BUFSIZE 1024
 char logbuf[BUFSIZE] = {0};