Fix test failures with no C extensions loaded.

* bzrlib/tests/test_selftest.py:
(TestActuallyStartBzrSubprocess.test_start_and_stop_bzr_subprocess_send_signal):Ignore
missing extensions.

* bzrlib/tests/blackbox/test_serve.py:
(TestBzrServe.setUp): Ignore missing extensions.

* bzrlib/tests/blackbox/test_locale.py:
Fix imports.
(TestLocale.test_log_C): Ignore missing extensions.

* bzrlib/tests/blackbox/test_cat.py:
Fix imports.
(TestCat.test_cat_different_id): Ignore missing extensions.