Fix writing timeout events. Check that we only return one timeout event.

crates/test-programs/wasi-tests/src/bin/poll_oneoff.rs

@@ -65,11 +65,17 @@ unsafe fn test_stdin_read() {
             r#type: wasi::EVENTTYPE_CLOCK,
             u: wasi::SubscriptionU { clock },
         },
+        // Make sure that timeout is returned only once even if there are multiple read events
         wasi::Subscription {
             userdata: 1,
             r#type: wasi::EVENTTYPE_FD_READ,
             u: wasi::SubscriptionU { fd_readwrite },
         },
+        wasi::Subscription {
+            userdata: 42,
+            r#type: wasi::EVENTTYPE_FD_READ,
+            u: wasi::SubscriptionU { fd_readwrite },
+        },
     ];
     let out = poll_oneoff_impl(&r#in, 1);
     let event = &out[0];