unit-tests: Further increase the test vector testing timeout

Some build bots running make check seem to have longer for the DH testing.

src/libstrongswan/tests/suites/test_vectors.c

@@ -60,7 +60,7 @@ Suite *vectors_suite_create()
 
 	tc = tcase_create("transforms");
 	tcase_add_loop_test(tc, test_vectors, 0, countof(tfs));
-	tcase_set_timeout(tc, 8);
+	tcase_set_timeout(tc, 20);
 	suite_add_tcase(s, tc);
 
 	return s;