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_xml.c b/tests/utests/test_xml.c
index 7710082..e9e22e1 100644
--- a/tests/utests/test_xml.c
+++ b/tests/utests/test_xml.c
@@ -24,8 +24,8 @@
 #include <stdio.h>
 #include <string.h>
 
-#include "../../src/xml.h"
-#include "../../src/context.h"
+#include "xml.h"
+#include "context.h"
 
 LY_ERR lyxml_ns_add(struct lyxml_ctx *xmlctx, const char *prefix, size_t prefix_len, char *uri);
 LY_ERR lyxml_ns_rm(struct lyxml_ctx *xmlctx);