Cleanup test imports and use features to better track skipped tests.

* bzrlib/tests/workingtree_implementations/__init__.py: 
Fix imports. Delete obsolete comment.

* bzrlib/tests/tree_implementations/test_walkdirs.py:
(TestWalkdirs.get_all_subdirs_expected): Reduce duplication.

* bzrlib/tests/tree_implementations/test_test_trees.py: 
Fix import.

* bzrlib/tests/tree_implementations/test_path_content_summary.py: 
Fix imports.

(TestPathContentSummary.test_unicode_symlink_content_summary,
TestPathContentSummary.test_unicode_symlink_target_summary):Use
UnicodeFilenameFeature instead of try/except UnicodeError.

* bzrlib/tests/tree_implementations/test_inv.py: 
Fix imports.
(TestInventoryWithSymlinks): Factor out test that requires
symlinks and use _test_needs_features.
(TestInventory.test_canonical_path,
TestInventory.test_canonical_path_dir,
TestInventory.test_canonical_path_root,
TestInventory.test_canonical_path_invalid_all,
TestInventory.test_canonical_invalid_child): Use assert(expected,
actual)

* bzrlib/tests/tree_implementations/test_get_symlink_target.py: 
Fix imports.
(TestGetSymlinkTarget.test_get_unicode_symlink_target): Use
UnicodeFilenameFeature instead of try/except UnicodeError.

* bzrlib/tests/tree_implementations/__init__.py: 
Fix imports.

(TestCaseWithTree.get_tree_with_subdirs_and_all_supported_content_types,
TestCaseWithTree._create_tree_with_utf8): Use
UnicodeFilenameFeature instead of try/except UnicodeError.

* bzrlib/tests/test_workingtree_4.py:
Fix too long lines.