tests/extmod: Add test for ujson.dump writing to a user IOBase object.

tests/extmod/ujson_dump_iobase.py

@@ -0,0 +1,32 @@
+# test ujson.dump in combination with uio.IOBase
+
+try:
+    import uio as io
+    import ujson as json
+except ImportError:
+    try:
+        import io, json
+    except ImportError:
+        print('SKIP')
+        raise SystemExit
+
+if not hasattr(io, 'IOBase'):
+    print('SKIP')
+    raise SystemExit
+
+
+# a user stream that only has the write method
+class S(io.IOBase):
+    def __init__(self):
+        self.buf = ''
+    def write(self, buf):
+        if type(buf) == bytearray:
+            # uPy passes a bytearray, CPython passes a str
+            buf = str(buf, 'ascii')
+        self.buf += buf
+
+
+# dump to the user stream
+s = S()
+json.dump([123, {}], s) 
+print(s.buf)