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/extensions/test_metadata.c b/tests/utests/extensions/test_metadata.c
index a97db00..f8353d2 100644
--- a/tests/utests/extensions/test_metadata.c
+++ b/tests/utests/extensions/test_metadata.c
@@ -12,8 +12,6 @@
  *     https://opensource.org/licenses/BSD-3-Clause
  */
 
-#include "tests/config.h"
-
 #include <stdarg.h>
 #include <stddef.h>
 #include <setjmp.h>
@@ -22,8 +20,8 @@
 #include <stdio.h>
 #include <string.h>
 
-#include "../../src/libyang.h"
-#include "../../src/plugins_exts_metadata.h"
+#include "libyang.h"
+#include "plugins_exts_metadata.h"
 
 #define BUFSIZE 1024
 char logbuf[BUFSIZE] = {0};