Make LocationConfig use a lock too.

* bzrlib/tests/test_config.py:
(lockable_config_scenarios, load_tests): What configs should be
tested as lockable ones.
(TestIniConfigBuilding.test_ini_config_ownership): Fixed.
(TestLockableConfig.get_existing_config): Some tests need to load
an existing config file rather than creating it.
(TestLockableConfig.test_read_while_writing): Ensure we get the
old config until the lock is released.