Use SKIPPED_TEST_EXIT_CODE consistently

Commit fa505bfb0e omitted two unit tests
while introducing the SKIP_TEST_EXIT_CODE preprocessor macro.  Fix the
outliers to make use of SKIP_TEST_EXIT_CODE consistent across all unit
tests.  Also make sure lib/dns/tests/dnstap_test returns an exit code
that indicates a skipped test when dnstap is not enabled.

lib/dns/tests/dnstap_test.c

@@ -382,7 +382,8 @@ main(void) {
 
 	return (cmocka_run_group_tests(tests, NULL, NULL));
 #else  /* if HAVE_DNSTAP */
-	print_message("1..0 # Skip dnstap not enabled\n");
+	print_message("1..0 # Skipped: dnstap not enabled\n");
+	return (SKIPPED_TEST_EXIT_CODE);
 #endif /* HAVE_DNSTAP */
 }