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/data/test_tree_data.c b/tests/utests/data/test_tree_data.c
index 0a1075c..9c38db0 100644
--- a/tests/utests/data/test_tree_data.c
+++ b/tests/utests/data/test_tree_data.c
@@ -18,7 +18,7 @@
 #include <setjmp.h>
 #include <cmocka.h>
 
-#include "../../src/libyang.h"
+#include "libyang.h"
 
 #define BUFSIZE 1024
 char logbuf[BUFSIZE] = {0};