test: fix tests to skip when missing required feature

test/unit/ntp_core.c

@@ -28,6 +28,8 @@
 #include <local.h>
 #include "test.h"
 
+#ifdef FEAT_NTP
+
 static struct timespec current_time;
 static NTP_Receive_Buffer req_buffer, res_buffer;
 static int req_length, res_length;
@@ -475,3 +477,11 @@ test_unit(void)
   CNF_Finalise();
   HSH_Finalise();
 }
+
+#else
+void
+test_unit(void)
+{
+  TEST_REQUIRE(0);
+}
+#endif