Implement 'bzr' command.
* bzrlib/tests/test_script.py:
(TestScriptExecution.test_unexpected_output): Test output semantic
change.
(TestBzr): Simple test for bzr command.
* bzrlib/tests/script.py:
(TestCaseWithScript.do_bzr): Implement the bzr command.
(TestCaseWithScript._check_output): Change output/error semantic:
if none is specified, no checking is done.