tests/extmod/ujson_dump_iobase.py: Return number of bytes written.
Otherwise returning None indicates that the write would block and nothing
was actually written.  Fixes issue
@@ -24,6 +24,7 @@ class S(io.IOBase):
# uPy passes a bytearray, CPython passes a str
buf = str(buf, 'ascii')
self.buf += buf
+ return len(buf)
# dump to the user stream
