clone_test_with_new_id shares details dict between test case instances

clone_test_with_new_id (or bzr's independent implementation, bzrlib.tests.clone_test) essentially just does a copy.copy of the test.

Unfortunately, the copy will share some attributes that should not be shared, especially __details and _cleanups.

In bzrlib's case this makes it difficult to read test failures when parameterised tests are involved — all parameterisations of a test share a details dict, so you might e.g. get 10 failing tests, each showing *10* tracebacks. In bzrlib's case it appears that a trivial __copy__ method that doesn't try to actually copy the instance but instead instantiates a new test case from scratch is an adequate workaround, but I'm not sure this is a good fix in general.

Perhaps clone_test_with_new_id should to be a method on TestCase? Or perhaps testtools.TestCase should provide a __copy__ and/or __deepcopy__ implementation.