Fix up SIGIO notifier tests

FreeBSD has the behavior where SIGIO is delivered on a read. Teach the
tests how to handle this behavior.

src/fish_tests.cpp

@@ -3867,6 +3867,12 @@ static void test_notifiers_with_strategy(universal_notifier_t::notifier_strategy
 
     // Nobody should poll now.
     for (size_t i = 0; i < notifier_count; i++) {
+        // On BSD, SIGIO may be delivered by read() even if it returns EAGAIN;
+        // that is the polling itself may trigger a SIGIO. Therefore we poll twice.
+        if (strategy == universal_notifier_t::strategy_sigio) {
+            (void)poll_notifier(notifiers[i]);
+        }
+
         if (poll_notifier(notifiers[i])) {
             err(L"Universal variable notifier polled true after all changes, with strategy %d",
                 (int)strategy);