tests/io/bytesio_ext: Test for .seek()/.flush() on BytesIO.

tests/io/bytesio_ext.py

@@ -0,0 +1,19 @@
+# Extended stream operations on io.BytesIO
+try:
+    import uio as io
+except ImportError:
+    import io
+
+a = io.BytesIO()
+print(a.seek(8))
+a.write(b"123")
+print(a.getvalue())
+
+print(a.seek(0, 1))
+
+print(a.seek(-1, 2))
+a.write(b"0")
+print(a.getvalue())
+
+a.flush()
+print(a.getvalue())