selftest --starting-with now accepts multiple values.

* bzrlib/tests/test_selftest.py:
(TestSelftestFiltering.test_condition_id_startswith)
(TestSelftestFiltering.test_filter_suite_by_id_startswith): Updated.

* bzrlib/tests/blackbox/test_selftest.py:
(TestSelftestStartingWith): Add test for multiple arguments.

* bzrlib/tests/__init__.py:
(condition_id_startswith, filter_suite_by_id_startswith): Now
accepts a list of strings to start with.
(test_suite): 'starting_with' is now a list.

* bzrlib/builtins.py:
(cmd_selftest): Allows multiple --starting-with option.