fix fragile test

Author: Guest0x0

src/io/writer_test.mbt

@@ -82,7 +82,7 @@ struct DummyWriter {
 
 ///|
 impl @io.Writer for DummyWriter with write_once(self, buf, offset~, len~) {
-  guard offset < len
+  guard offset + len <= buf.length()
   @async.sleep(self.base_timeout)
   self.logger.write_string("writing byte \{buf[offset]}\n")
   1
@@ -92,8 +92,8 @@ impl @io.Writer for DummyWriter with write_once(self, buf, offset~, len~) {
 async test "write cancel" {
   let log = StringBuilder::new()
   @async.with_task_group(fn(_) {
-    let writer = { logger: log, base_timeout: 40 }
-    @async.with_timeout_opt(60, fn() { writer.write(b"abcd") }) |> ignore
+    let writer = { logger: log, base_timeout: 300 }
+    @async.with_timeout_opt(450, fn() { writer.write(b"abcd") }) |> ignore
   })
   // The `write` here is partial and corrupted,
   // but we have no choice because making `write` atomic may cause hang