Provide a better implementation for testing passwords.

* bzrlib/ui/__init__.py:
(UIFactory.get_login): Deleted.
(CLIUIFactory.get_non_echoed_password): New method allowing
overriding.

* bzrlib/tests/__init__.py:
(TestUIFactory.get_non_echoed_password): Allows password testing
 without worrying about echo echo.

* bzrlib/tests/__init__.py:
(TestUIFactory): Moved from bzrlib/tests/blackbox/__init__.py
(FakeStdin): Deleted.
(TestCase.run_bzr_captured): Set and reuse ui.ui_factory.stdin.

* bzrlib/ui/text.py:
(TextUIFactory.get_login): Deleted.
(TextUIFactory.get_password): Moved to CLIUIFactory.

* bzrlib/tests/test_ui.py:
(UITests): Delete get_login tests.
(FakeTextUIFactory): Deleted. Better implementation in
TestUIFactory.

* bzrlib/tests/blackbox/__init__.py:
(TestUIFactory): Moved to bzrlib/tests/__init__.py.