Implemented asyncfile for Posix.

tests/async/tasyncfile.nim

@@ -5,10 +5,32 @@ discard """
 import asyncfile, asyncdispatch, os
 
 proc main() {.async.} =
-  var file = openAsync(getTempDir() / "foobar.txt", fmReadWrite)
-  await file.write("test")
-  file.setFilePos(0)
-  let data = await file.readAll()
-  doAssert data == "test"
+  let fn = getTempDir() / "foobar.txt"
+  removeFile(fn)
 
+  # Simple write/read test.
+  block:
+    var file = openAsync(fn, fmReadWrite)
+    await file.write("test")
+    file.setFilePos(0)
+    await file.write("foo")
+    file.setFilePos(0)
+    let data = await file.readAll()
+    doAssert data == "foot"
+    file.close()
+  
+  # Append test
+  block:
+    var file = openAsync(fn, fmAppend)
+    await file.write("\ntest2")
+    let errorTest = file.readAll()
+    await errorTest
+    doAssert errorTest.failed
+    file.close()
+    file = openAsync(fn, fmRead)
+    let data = await file.readAll()
+    
+    doAssert data == "foot\ntest2"
+    file.close()
+  
 waitFor main()