Fix OSX and FreeBSD failures.

* bzrlib/tests/__init__.py:
(TestCaseWithMemoryTransport._make_test_root): Fixing the problem
at the root, if we usr the real path there, we catch 99% of the
consequences.

* bzrlib/tests/test_osutils.py:
(TestCanonicalRelPath.test_canonical_relpath_simple,
TestCanonicalRelPath.test_canonical_relpath_missing_tail): No need
to use realpath here anymore.

* bzrlib/tests/script.py:
(ScriptRunner.do_rm): OSX and BSD raises a different exception,
and windows too even if that will require real testing (but
osutils._delete_file_or_dir use the same trick for unlink so it's
a pretty safe bet).

* bzrlib/tests/blackbox/test_outside_wt.py:
(TestOutsideWT.test_cwd_log,
TestOutsideWT.test_diff_outside_tree): Watch for OSX trick.