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/schema/test_schema.c b/tests/utests/schema/test_schema.c
index ffb7195..9a03332 100644
--- a/tests/utests/schema/test_schema.c
+++ b/tests/utests/schema/test_schema.c
@@ -22,9 +22,9 @@
 
 #include <string.h>
 
-#include "config.h"
 #include "log.h"
 #include "parser_schema.h"
+#include "tests/config.h"
 #include "tree_schema.h"
 
 #if ENABLE_LOGGER_CHECKING