tests: Add test for io.BufferedWriter.

tests/io/buffered_writer.py

@@ -0,0 +1,22 @@
+import _io as io
+
+try:
+    io.BytesIO
+    io.BufferedWriter
+except AttributeError:
+    import sys
+    print('SKIP')
+    sys.exit()
+
+bts = io.BytesIO()
+buf = io.BufferedWriter(bts, 8)
+
+buf.write(b"foobar")
+print(bts.getvalue())
+buf.write(b"foobar")
+# CPython has different flushing policy, so value below is different
+print(bts.getvalue())
+buf.flush()
+print(bts.getvalue())
+buf.flush()
+print(bts.getvalue())

tests/io/buffered_writer.py.exp

@@ -0,0 +1,4 @@
+b''
+b'foobarfo'
+b'foobarfoobar'
+b'foobarfoobar'