os: Add simple read/write test.

Author: pfalcon

os/test_rw.py

@@ -0,0 +1,6 @@
+import os
+
+
+assert os.write(1, "hello\n") == len("hello\n")
+data = os.read(0, 256)
+os.write(2, "read: %s\n" % data)