Add test for file write and seek

tests/wasi-fyi/fs_write-and-seek.rs

@@ -0,0 +1,39 @@
+use std::fs::{metadata, OpenOptions};
+use std::io::{Seek, SeekFrom, Write};
+
+fn main() {
+    let mut file = OpenOptions::new()
+        .write(true)
+        .read(true)
+        .create(true)
+        .open("file")
+        .unwrap();
+
+    write!(file, "hell").unwrap();
+
+    // We wrote 4 bytes
+    let md = metadata("file").unwrap();
+    assert_eq!(md.len(), 4);
+
+    assert_eq!(file.seek(SeekFrom::Start(0)).unwrap(), 0);
+
+    write!(file, "eh").unwrap();
+
+    // We overwrote the first 2 bytes, should still have 4 bytes
+    let md = metadata("file").unwrap();
+    assert_eq!(md.len(), 4);
+
+    assert_eq!(file.seek(SeekFrom::Start(0)).unwrap(), 0);
+
+    write!(file, "hello").unwrap();
+
+    // Now we wrote past the end, should have 5 bytes
+    let md = metadata("file").unwrap();
+    assert_eq!(md.len(), 5);
+
+    write!(file, " world!").unwrap();
+
+    // We wrote past the end entirely, should have 5 + 7 = 12 bytes
+    let md = metadata("file").unwrap();
+    assert_eq!(md.len(), 12);
+}