[PATCH] externals_tests.py should handle missing local externals item

Hi!

[[[
When updating to loose an external the code should handle a locally
missing externals item.

* subversion/tests/cmdline/externals_tests.py
  (update_loose_external): Remove one externals wc dir.
]]]

I'm writing a patch to remove some adm_access batons. That patch made
the externals_tests even though it didn't handle missing local
externals which trunk does.

Why should we handle a locally missing externals here? The user might
have thought that the externals items should be removed manually after
beeing removed from the property. Users do strange things. Handle it
gracefully!

I thought the change was so small that it could be included
in an existing test case.

Daniel