Add condition_id_in_list and filter_suite_by_id_list capabilities.

* bzrlib/tests/test_selftest.py:
(TestSelftestFiltering.test_condition_id_in_list,
TestSelftestFiltering.test_filter_suite_by_id_list): Tests for
filtering test suite by test id.

* bzrlib/tests/__init__.py:
(filter_suite_by_id_list, condition_id_in_list): New test
filtering helpers to select tests by their id.