Introduce BZR_COLUMNS since COLUMNS behaviour is too obscure.
* bzrlib/tests/test_osutils.py:
(TestTerminalWidth): Update tests.
* bzrlib/tests/__init__.py:
(TestCase._cleanEnvironment): Add BZR_COLUMNS.
* bzrlib/osutils.py:
(terminal_width): BZR_COLUMNS is the official way to override.