Fix nondeterministic failures in poll_oneoff_stdio.

Adjust this test so that it tolerates poll_oneoff returning that both a
timeout occurred and an input is ready for reading, at the same time.

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

@@ -46,8 +46,8 @@ unsafe fn test_stdin_read() {
     let out = poll_oneoff_impl(&r#in).unwrap();
     // The result should be either a timeout, or that stdin is ready for reading.
     // Both are valid behaviors that depend on the test environment.
-    assert_eq!(out.len(), 1, "should return 1 event");
-    let event = &out[0];
+    assert!(out.len() >= 1, "should return at least 1 event");
+    for event in out {
         if event.r#type == wasi::EVENTTYPE_CLOCK {
             assert_errno!(event.error, wasi::ERRNO_SUCCESS);
             assert_eq!(
@@ -63,6 +63,7 @@ unsafe fn test_stdin_read() {
         } else {
             panic!("unexpected event type {}", event.r#type);
         }
+    }
 }
 
 unsafe fn test_stdout_stderr_write() {