Use BIND 9.17 preprocessor macro to skip unit test

BIND 9.17 changed exit code of skipped test to meet Automake
expectations in fa505bfb0e. BIND 9.16 was
not rewritten to Automake, but for consistency reasons, the same
SKIPPED_TEST_EXIT_CODE preprocessor macro is used (though the actual
exit code differs from the one in BIND 9.17).

(cherry picked from commit fa505bfb0e)

lib/ns/tests/plugin_test.c

@@ -201,7 +201,7 @@ main(void) {
 int
 main(void) {
 	printf("1..0 # Skipped: cmocka not available\n");
-	return (0);
+	return (SKIPPED_TEST_EXIT_CODE);
 }
 
 #endif /* if HAVE_CMOCKA */