golang: tests: Verify that b and u are idempotent

i.e. b(b(·)) and u(u(·)) are always identity.

This property already holds and is easy to verify just by code review.
However it might become less obvious once we start to tweak b and u.

-> Add explicit test.

golang/golang_test.py

@@ -30,6 +30,7 @@ from os.path import dirname
 import os, sys, inspect, importlib, traceback, doctest
 from subprocess import Popen, PIPE
 import six
+from six import text_type as unicode
 from six.moves import range as xrange
 import gc, weakref, warnings
 import re
@@ -1640,6 +1641,16 @@ def test_strings():
 
     # TODO also handle bytearray?
 
+    # b(b(·)) = identity
+    _ = b(u'миру мир 123')
+    assert isinstance(_, bytes)
+    assert b(_) is _
+
+    # u(u(·)) = identity
+    _ = u(u'мир труд май')
+    assert isinstance(_, unicode)
+    assert u(_) is _
+
 
 # ---- misc ----