Oh dear, I was missing an extern.

src/test/test.h

@@ -169,7 +169,7 @@ struct crypto_pk_t *pk_generate(int idx);
 #define NS_UNMOCK(name) UNMOCK(name)
 
 extern const struct testcase_setup_t passthrough_setup;
-const struct testcase_setup_t ed25519_test_setup;
+extern const struct testcase_setup_t ed25519_test_setup;
 
 extern struct testcase_t accounting_tests[];
 extern struct testcase_t addr_tests[];