Make sure to sync on file-io.rs tokio test

Tokio `AsyncWriteExt::write` doesn't actually ensure that the contents
have written, it just *starts* the write operation. To ensure that the
file has actually been written, we need to `sync_all` first.

src/tools/miri/tests/pass-dep/tokio/file-io.rs

@@ -21,6 +21,7 @@ async fn test_create_and_write() -> io::Result<()> {
 
     // Write 10 bytes to the file.
     file.write_all(b"some bytes").await?;
+    file.sync_all().await?; // tokio doesn't necessarily complete writes until you sync.
     assert_eq!(file.metadata().await.unwrap().len(), 10);
 
     remove_file(&path).unwrap();