Deprecate the varargs syntax and fix the tests.

* bzrlib/tests/__init__.py:
(TestCase.run_bzr): Activate the deprecation warning.
(TestCase.run_bzr_error): Add error_regexes to kwargs or run_bzr
get confused.

* bzrlib/tests/blackbox/test_selftest.py:
(TestRunBzr.test_args): Activate.

* bzrlib/tests/blackbox/test_inventory.py:
(TestInventory.assertInventoryEqual): Build the command from args
if not None.

* bzrlib/tests/blackbox/test_ls.py:
(TestLS.ls_equals): Build the command from args if not None.

* bzrlib/tests/blackbox/test_remove_tree.py:
(TestRemoveTree.test_remove_tree_lightweight_checkout_explicit):
Nice catch, we were calling run_bzr instead of run_bzr_error. This
went unnoticed for some time...