Use assertIsSameRealPath to avoid OSX aliasing (specifically /tmp
-> /private/tmp).

* bzrlib/tests/__init__.py:
(TestCase.assertIsSameRealPath): New method resolving symlinks
before comparing paths.

* bzrlib/tests/test_selftest.py:
(TestTestCaseInTempDir.test_home_is_not_working,
TestTestCaseWithMemoryTransport.test_home_is_non_existant_dir_under_root,
TestTestCaseWithMemoryTransport.test_cwd_is_TEST_ROOT): Use
assertIsSameRealPath.

* bzrlib/tests/test_plugins.py:
(TestPlugins.test_trivial_plugin_get_path): Use assertIsSameRealPath.

* bzrlib/tests/test_sftp_transport.py:
(SFTPTransportTestRelative.test__remote_path): The test should be
totally rewritten for OSX. Marking it as know failure in the
interim.