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 #3990.

tests/extmod/ujson_dump_iobase.py

@@ -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