Adds test case for #6846. (#8729)

tests/async/t6846.nim

@@ -0,0 +1,16 @@
+discard """
+  exitcode: 0
+  output: "hello world"
+  disabled: windows
+"""
+
+import asyncdispatch
+import asyncfile
+import times
+
+var asyncStdout = 1.AsyncFD.newAsyncFile()
+proc doStuff: Future[void] {.async.} =
+  await asyncStdout.write "hello world\n"
+
+let fut = doStuff()
+doAssert fut.finished, "Poll is needed unnecessarily. See #6846."