[MERGE] Improvements to test_bzrdir

[Andrew Bennetts]

This is a bunch of changes I've made to test_bzrdir.py in the process of working
on the smart server with Martin and Robert.  It does the following things:

 * Skip tests that require a local transport when we aren't testing with one.
   This helps the smart server work, but can also be seen with "bzr selftest
   --transport sftp".
 * Test that directories exist using found_transport.list_dir rather than stat
   (it's more reliable and clear).
 * Fixes test method name clash -- there were two methods named
   "test_sprout_bzrdir_empty_under_shared_repo"
 * Fixes bug in BzrDir.sprout revealed by fixing the test name clash

-Andrew.

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