Fixed as per review comments.
* bzrlib/tests/test_plugins.py:
(TestDisablePlugin.test_not_loaded,
TestDisablePlugin.test_cannot_import): Update tests.
* bzrlib/tests/__init__.py:
(TestCase._cleanEnvironment): Add BZR_DISABLE_PLUGINS.
* bzrlib/plugin.py:
(set_plugins_path): Use a dedicated BZR_DISABLE_PLUGINS env var
instead of mucking with BZR_PLUGIN_PATH.