* bzrlib/tests/test_crash.py: (TestApportDeprecation): Reproduce bug #516934 leading to an infinite recursion when apport is available.
* bzrlib/tests/test_selftest.py: (simple_thunk_feature): Use the new signature for _CompatabilityThunkFeature.
* bzrlib/tests/features.py: (ApportFeature): Use the abbreviated name instead of the full symbol name.
* bzrlib/tests/__init__.py: (_CompatabilityThunkFeature.__init__): Use clearer parameter names to specify the feature and its replacement version. Default to the same module for both features. (_CompatabilityThunkFeature._ensure): Really thunk to the replacement feature. Emits two distinct messages for easier testing.