tests: fix warnings

While I don't see -Werror being used on tests anytime soon, there's no
reason to keep the warnings in tests unfixed.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/tests/heavy-thread.c b/tests/heavy-thread.c
index cd3a3b9..c2e71c1 100644
--- a/tests/heavy-thread.c
+++ b/tests/heavy-thread.c
@@ -37,6 +37,8 @@
 #include "memory.h"
 #include "log.h"
 
+#include "tests.h"
+
 extern struct thread_master *master;
 
 enum