Relax constraint on test ids, simplify implementation and update tests.

* bzrlib/tests/test_selftest.py:
(TestSelftestFiltering.test_condition_id_in_list,
TestSelftestFiltering.test_filter_suite_by_id_list): Updated.
(TestTestIdList): Renamed from TestTestIdListFilter and updated.

* bzrlib/tests/__init__.py:
(TestIdList): Relax the constraint on test ids, they just have to
start with their module name.

* bzrlib/doc/api/__init__.py:
(test_suite): Adding module name in test id is now enough.